[nit] command_line.rst: the standard format for multiple choice has no space (#19868)

Removing this spaces causes this documentation to match the other
documentation on the page, and also in `--help`

There are no tests for this change. I manually verified that the link to
the option currently is
https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-enable-incomplete-feature,
so changing the arguments text does not necessitate adding a new anchor
in to preserve old inbound links to this section.

Author: wyattscarpenter

docs/source/command_line.rst

@@ -1159,7 +1159,7 @@ format into the specified directory.
 Enabling incomplete/experimental features
 *****************************************
 
-.. option:: --enable-incomplete-feature {PreciseTupleTypes, InlineTypedDict}
+.. option:: --enable-incomplete-feature {PreciseTupleTypes,InlineTypedDict}
 
     Some features may require several mypy releases to implement, for example
     due to their complexity, potential for backwards incompatibility, or