cmd/gopherbot: rename a function to be more descriptive
cmd/gopherbot: rename a function to be more descriptive

labelChangeDisallowed is closer to what this function is meant for.

Change-Id: I64b143263df2d3dca018f9daecd73c2be59e84c9
Reviewed-on: https://go-review.googlesource.com/c/build/+/260417
Trust: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build