docs: clarify that libfaketime requires dynamically linked libc

[LIJOIGNATIUS]

• Updated README to specify that libfaketime requires libc to be dynamically linked, rather than stating the entire binary must be dynamically linked
• Fixed typo: "you system linker's" → "your system linker's"

[LIJOIGNATIUS]

Hi! Interesting library — I just started going through the code and documentation, and while reading, I noticed this wording could be a bit clearer, so I tried a small doc contribution. Thanks for maintaining this project!