Update README.rst correcting grammar.
Update README.rst correcting grammar.

`there is several` should be `there are several`
xavfernandez/pip,pfmoore/pip,pypa/pip,sbidoul/pip,xavfernandez/pip,xavfernandez/pip,pfmoore/pip,sbidoul/pip,pradyunsg/pip,pypa/pip,pradyunsg/pip