Use British English spelling

[AkmalFairuz]

In CONTRIBUTING.md:

Make sure to use British English and proper punctuation throughout symbol names, variables and documentation.

Updated comments, type names, and documentation to use British English spellings, such as "armour" instead of "armor", "customise" instead of "customize", "centre" instead of "center", "initialise" instead of "initialize", "behaviour" instead of "behavior", and "defence" instead of "defense"