Convert docs to American English per new Canonical style

[benhoyt]

I thought there was an issue open for this already, but I couldn't find one. We should update the Ops docs to the new Canonical docs recommendation (American English instead of British) and remove the related paragraph from our STYLE.md.

See also #1720.

[dwilding]

Thanks Ben

We have this similar one for Pebble: https://github.com/canonical/pebble/issues/562

[benhoyt]

@dimaqq Volunteered! :-)

[dimaqq]

Volunteered or voluntold?