gopls/doc/troubleshooting.md: update the gopls output channel name
gopls/doc/troubleshooting.md: update the gopls output channel name

Since v0.15.0, VS Code Go extension uses the `gopls (server)` channel
for the server-side logging.

Change-Id: I6dc2125af59562576d20e29a0801be6b4ce394bb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/240677
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools