Fix minor typo in attribute docs.

[jeongukjae]

What type of PR is this?

Documentation

What does this PR do? Why is it needed?

Fix minor typos. Actual valid choices for these options are "auto", "on", "off", but the docs says `on`, `on`, or `auto`.

Which issues(s) does this PR fix?

Fixes # Skipping this section.

Other notes for review

I checked the choices of the attrs in below links.

• https://github.com/bazelbuild/rules_go/blob/bd44f4242b46e73fb2a81fc87ea4b52173bde84e/go/private/rules/transition.bzl#L97-L100
• https://github.com/bazelbuild/rules_go/blob/bd44f4242b46e73fb2a81fc87ea4b52173bde84e/go/private/rules/transition.bzl#L93-L96

Thanks :)

[robfig]

Interesting, it looks like there's a test for the documentation that needs to be updated?
//docs:docs_go_core_rules-difftest

[jeongukjae]

@robfig Oh, thank you :)
I thought this is a just fix for the docs, so I didn't check the test cases. I ran bazel run //docs:update.

It seems all tests are passed now.

[robfig]

Thank you!