Fix multiline f-string formatting

The previously used f-string format did not properly interpolate the
variables, and resulted in an output such as the following:

```
/Users/username/cpython/Lib/multiprocessing/resource_tracker.py:224: UserWarning: resource_tracker: There appear to be 6 leaked {rtype} objects to clean up at shutdown: {rtype_cache}
```

Resource: https://stackoverflow.com/questions/49416042/how-to-write-an-f-string-on-multiple-lines-without-introducing-unintended-whites

Lib/multiprocessing/resource_tracker.py

@@ -223,7 +223,7 @@ def main(fd):
                 try:
                     warnings.warn(
                         f'resource_tracker: There appear to be {len(rtype_cache)} '
-                        'leaked {rtype} objects to clean up at shutdown: {rtype_cache}'
+                        f'leaked {rtype} objects to clean up at shutdown: {rtype_cache}'
                     )
                 except Exception:
                     pass