Fix reference to "gem" in README

Fix a reference to a "gem" in README and link to the RubyGems gem. Point
to the PyPI package instead (although we're not quite yet published
there).

Author: brandur

README.md

@@ -1,6 +1,6 @@
 # River client for Python
 
-An insert-only Python client for [River](https://github.com/riverqueue/river) packaged in the [`riverqueue` gem](https://rubygems.org/gems/riverqueue). Allows jobs to be inserted in Python and run by a Go worker, but doesn't support working jobs in Python.
+An insert-only Python client for [River](https://github.com/riverqueue/river) packaged in the [`riverqueue` package on PyPI](https://pypi.org/project/riverqueue/). Allows jobs to be inserted in Python and run by a Go worker, but doesn't support working jobs in Python.
 
 ## Development