doc: remove duplicate word (#16365)

This PR removes one of the duplicate **in** in the sentence "This option
is only useful in in the absence of `__init__.py`" in the file
`docs/source/command_line.rst`.

Author: dinaldoap

docs/source/command_line.rst

@@ -133,7 +133,7 @@ imports.
 
     This flag tells mypy that top-level packages will be based in either the
     current directory, or a member of the ``MYPYPATH`` environment variable or
-    :confval:`mypy_path` config option. This option is only useful in
+    :confval:`mypy_path` config option. This option is only useful
     in the absence of `__init__.py`. See :ref:`Mapping file
     paths to modules <mapping-paths-to-modules>` for details.