Fixed cargo install line in README

[Floppy]

I think the cargo install command was wrong, it was referencing matrix.org, not github, and when I ran it I got 404s.

[AlphaCraft9658]

When I try running this with a recent version of cargo, and I have actually used a rust:slim docker container for the build, the build fails with the following six errors:

error[E0599]: no method named `get_joined_room` found for struct `matrix_sdk::Client` in the current scope
   --> src/main.rs:202:39
    |
202 |             let Some(joined) = from_c.get_joined_room(&room_id) else {
    |                                       ^^^^^^^^^^^^^^^
    |
help: there is a method `get_room` with a similar name
    |
202 -             let Some(joined) = from_c.get_joined_room(&room_id) else {
202 +             let Some(joined) = from_c.get_room(&room_id) else {
    |

error[E0599]: no method named `get_invited_room` found for reference `&matrix_sdk::Client` in the current scope
   --> src/main.rs:245:34
    |
245 |         let Some(invited) = to_c.get_invited_room(&room_id) else {
    |                                  ^^^^^^^^^^^^^^^^
    |
help: there is a method `get_room` with a similar name
    |
245 -         let Some(invited) = to_c.get_invited_room(&room_id) else {
245 +         let Some(invited) = to_c.get_room(&room_id) else {
    |

error[E0599]: no method named `get_joined_room` found for reference `&matrix_sdk::Client` in the current scope
   --> src/main.rs:246:21
    |
246 |             if to_c.get_joined_room(room_id).is_some() { // already existing, skipping
    |                     ^^^^^^^^^^^^^^^
    |
help: there is a method `get_room` with a similar name
    |
246 -             if to_c.get_joined_room(room_id).is_some() { // already existing, skipping
246 +             if to_c.get_room(room_id).is_some() { // already existing, skipping
    |

error[E0599]: no method named `get_joined_room` found for struct `matrix_sdk::Client` in the current scope
   --> src/main.rs:273:39
    |
273 |             let Some(joined) = from_c.get_joined_room(&room_id) else {
    |                                       ^^^^^^^^^^^^^^^
    |
help: there is a method `get_room` with a similar name
    |
273 -             let Some(joined) = from_c.get_joined_room(&room_id) else {
273 +             let Some(joined) = from_c.get_room(&room_id) else {
    |

error[E0599]: no method named `login_username` found for struct `matrix_sdk::Client` in the current scope
  --> src/main.rs:66:10
   |
65 | /     from_c
66 | |         .login_username(args.from_user, &args.from_user_password)
   | |         -^^^^^^^^^^^^^^ method not found in `matrix_sdk::Client`
   | |_________|
   |

error[E0599]: no method named `login_username` found for struct `matrix_sdk::Client` in the current scope
  --> src/main.rs:82:10
   |
82 |     to_c.login_username(args.to_user, &args.to_user_password)
   |          ^^^^^^^^^^^^^^ method not found in `matrix_sdk::Client`

Even with this fixed command, the build no longer works.
It appears that the methods get_joined_room and get_invited_room have been removed or replaced in later versions of the matrix-sdk.