Update directory name in install.md

[Stigjb]

The directory name does not match the current version number. Update the directory name and update the example to use the concrete path instead of the <spgroot> placeholder.

[Stigjb]

I could mention that this patch is motivated by a student who got stuck on this step because writing a literal <spgroot> in the command did not work. On the other hand, I believe anyone with the experience to realize that <spgroot> should be replaced by the actual name of the directory, would also change the directory name as appropriate if they need to use a different version than spgl1-2.1.