Correction to execution command

The command
```sh
node --inspect node_modules/@google-cloud/functions-framework --target=helloWorld
```
didn't work for me. Not sure if this is because of changes, npm updates or the doc was wrong in the first place.

The working command is documented in this change.

Author: gg-ayushka

docs/debugging.md

@@ -19,7 +19,7 @@ npm install @google-cloud/functions-framework
 3. Run `node`, enable the inspector and run the Functions Framework:
 
 ```sh
-node --inspect node_modules/@google-cloud/functions-framework --target=helloWorld
+node --inspect node_modules/.bin/functions-framework --target=helloWorld
 ...
 Debugger listening on ws://127.0.0.1:9229/5f57f5e9-ea4b-43ce-be1d-6e9b838ade4a
 For help see https://nodejs.org/en/docs/inspector