README: make install command a code block

[Cube707]

Here we go again 😆

You have greate repos...

[asottile]

via:

all-repos-sed \
    --branch-name pip-install-block \
    --commit-msg $'convert pip install comand to a block\n\ngithub will produce a copy-paste button' \
    -r 's/^`(pip install .*)`$/```bash\n\1\n```/g' README.md \
    --repos $(all-repos-grep --repos '^`pip install' -- README.md)