Update testing-sources.md

The current syntax does not work on a mac terminal using zshell.

website/docs/faqs/Tests/testing-sources.md

@@ -9,7 +9,7 @@ id: testing-sources
 To run tests on all sources, use the following command:
 
 ```shell
-$ dbt test --select source:*
+$ dbt test --select "source:*"
 ```
 
 (You can also use the `-s` shorthand here instead of `--select`)