docs(stats): update get_stats() example to current output (worker keys + counts)

[pacocartones]

Fix the get_stats() example in the Stats docs

The example output under "Get Stats" in docstrings/stats.rst is out of date. Since
the multiprocessing support landed in 9.1.0, get_stats() also returns four
aggregated worker counters that are always present (zeroed on a serial run):

• WORKER DIFF COUNT
• WORKER PASSES COUNT
• WORKER DISTANCE CACHE HIT COUNT
• WORKER BATCH COUNT

The documented example still shows the pre-9.1.0 dict, so it neither lists those keys
nor matches the current counter values. Running the example exactly as written now
gives:

>>> pprint(diff.get_stats())
{'DIFF COUNT': 54,
 'DISTANCE CACHE HIT COUNT': 9,
 'MAX DIFF LIMIT REACHED': False,
 'MAX PASS LIMIT REACHED': False,
 'PASSES COUNT': 7,
 'WORKER BATCH COUNT': 0,
 'WORKER DIFF COUNT': 0,
 'WORKER DISTANCE CACHE HIT COUNT': 0,
 'WORKER PASSES COUNT': 0}

This updates the example to that output. Docs-only change; the WORKER * keys being
present in serial mode is the intended behaviour and is already covered by
tests/test_multiprocessing.py.

(docs/stats.rst is a symlink to deepdiff/docstrings/stats.rst, so the single edit
covers both.)