docs: context-only label is wrong/misleading (should be just `context`)

[kemcbride]

Hey - small thing I noticed while setting up sloglint at work - the usage in the README says it's context-only for turning on the context linter, but actually it's just context.

Not a huge deal but figured I'd drop an issue for it!

[tmzane]

Thanks! I'll sync the docs with golangci-lint.