Fix missing space in option description (#4182)

src/black/__init__.py

@@ -279,7 +279,7 @@ def validate_regex(
  is_flag=True,
  help=(
  "Format all input files like Jupyter Notebooks regardless of file extension."
- "This is useful when piping source on standard input."
+ " This is useful when piping source on standard input."
  ),
 )
 @click.option(