[github_activity] Readme.rst has wrong reference to library used by plugin

[silpol]

in the very top of Readme.rst there is link to feedparser. However, that URL https://crate.io/packages/feedparser gives 404. Even assuming author wanted to link to crates.io as lib written in Rust - that URL https://crates.io/packages/feedparser also appears to be 404 too.
As expected, feedparser is present in PyPi and seem to be what we need for plugin.