Americanise spellings

[cbracken]

Description

Updates documentation and non-public API to use American spellings for
consistency with the rest of the codebase.

No changes to behaviour... other than how it's spelt.

Made use of misspell tool:
https://github.com/client9/misspell

Related Issues

n/a

Tests

No tests added.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[xster]

LGTM