Fix small comment typo (#2112)

We probably don't need to fall back on "polling" when setting up an asyncio loop

src/black/__init__.py

@@ -784,7 +784,7 @@ def reformat_many(
     except (ImportError, OSError):
         # we arrive here if the underlying system does not support multi-processing
         # like in AWS Lambda or Termux, in which case we gracefully fallback to
-        # a ThreadPollExecutor with just a single worker (more workers would not do us
+        # a ThreadPoolExecutor with just a single worker (more workers would not do us
         # any good due to the Global Interpreter Lock)
         executor = ThreadPoolExecutor(max_workers=1)