Fix a comment that misrepresented the Assign operator (=).

Rename: colon-equals -> equals.

Change-Id: I405b8acfb0bcd1b176a91a95f9bfb61a4e85815f
GitHub-Last-Rev: aec0bf594c63d7b015f88f97f9953ade976817a4
GitHub-Pull-Request: golang/go#26112
Reviewed-on: https://go-review.googlesource.com/121416Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>