docs/contributing.md: remove unnecessary quote

There is an extra quote in the command, which should be an error.

Change-Id: I6ff7c00fa7cacaaef9476232637bff110647ff40
GitHub-Last-Rev: 703b7a1
GitHub-Pull-Request: #3042
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/541875
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
Commit-Queue: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>

Author: rogeryk

docs/contributing.md

@@ -125,7 +125,7 @@ When developing features in `gopls`, you may need to attach a debugger to `gopls
 
 1. Start a gopls in deamon mode:
 ```
-gopls -listen=:37374" -logfile=auto -debug=:0 serve
+gopls -listen=:37374 -logfile=auto -debug=:0 serve
 ```
 
 Or, if you use vscode for gopls development, you can configure `launch.json` of the `x/tools/gopls` project: