doc: update library generation howto to use module path (googleapis#16034)

Author: scotthart

doc/contributor/howto-guide-adding-generated-libraries.md

@@ -97,7 +97,7 @@ and add the new service.
 Find the list of `.proto` files that will need to be included:
 
 ```shell
-find "${bazel_output_base}/external/googleapis~/${subdir}" -name '*.proto' -print0 |
+find "${bazel_output_base}/external/googleapis+/${subdir}" -name '*.proto' -print0 |
   xargs -0 grep -l '^service'
 ```