DOC: s/xasis/xaxis/ in whats_new.rst

[akrherz]

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[tacaswell]

Thanks!

Just want to wait for CI to spin before merging.

[dstansby]

The travis 3.6 test seems to be flaky, but it's not related to this.

[QuLogic]

@Carreau sorry to ping you again, but @dstansby is on the whitelist, no?

[Carreau]

Yes.

[Carreau]

I've invited you on the heroku app, so you can see the logs.

[QuLogic]

@meeseeksdev backport to v2.2.x