Fix the missing closing backticks in CONTRIBUTING (google-gemini#514)

The rendering is broken due to the missing closing backticks.

Author: rakyll

CONTRIBUTING.md

@@ -93,10 +93,11 @@ pytest
 
 Or to debug, use:
 
-```commandline
+```
 pip install nose2
 
 nose2 --debugger
+```
 
 ### Type checking 
 
@@ -124,7 +125,6 @@ black .
 python docs/build_docs.py
 ```
 
-
 [setup]: https://cloud.google.com/nodejs/docs/setup
 [projects]: https://console.cloud.google.com/project
 [billing]: https://support.google.com/cloud/answer/6293499#enable-billing