[gitignore] restore the old Git behavior for the default modules (#3968)

The pattern `modules` was too broad and prevented tracking files in
`modules/default/` despite the negation pattern. Changed to `modules/*`
to properly exclude only the content of the modules directory while
allowing the default modules to be tracked.

This issue was likely introduced during the cleanup in #3952.

Without this change there are now warn messages like this:

```bash
kristjan@debian:~/MagicMirror$ git add modules/default/compliments/compliments.js
The following paths are ignored by one of your .gitignore files:
modules
hint: Use -f if you really want to add them.
hint: Disable this message with "git config advice.addIgnoredFile false"
```

.gitignore

@@ -55,7 +55,7 @@ Temporary Items
 .Trash-*
 
 # Ignore all modules except the default modules.
-modules
+modules/*
 !modules/default
 
 # Ignore changes to the custom css files but keep the sample and main.