There may be some subtlety I'm not seeing with the usage
of center vs centre. Otherwise this makes it more consistent
in the docs.
There's one instance in 8.0.2-notes.rst, but I'm not sure if
we want to modify it retroactively.

Test Plan: harbormaster?

Reviewers: bgamari, austin

Reviewed By: bgamari

Subscribers: rwbarton, thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D3548