docs: clarify single-word Go naming conventions

[Vishal202-rgb]

Summary

Clarifies the naming convention for single-word identifiers in the Go Style Guide.

Changes

• Clarified that single-word unexported names begin with a lowercase letter.
• Clarified that single-word exported names begin with an uppercase letter.
• Kept the clarification consistent with the existing capitalization rules in the MixedCaps section.

Issue

Fixes #940

Testing

• Reviewed the resulting documentation diff.
• Ran git diff --check successfully with no whitespace errors.
• Verified that only go/guide.md was modified.

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[KatrinaHoffert]

Hi, thanks for your contribution, but I'm afraid per https://github.com/google/styleguide#contributing, we don't accept external contributions. We don't mention anything on single word names because it's enforced by Go as a language and is more or less standard for camelCase, so there's no real style implications.