Broken link to the commit hash in the latest documentation

[seisman]

To reproduce the issue:

1. Go to https://www.pygmt.org/latest/
2. Scroll down to the bottom
3. Click the "Revision v0.3.1" link:

"v0.3.1" is linked to a nonexistent commit hash (https://github.com/GenericMappingTools/pygmt/commit/v0.3.1).