Correct "systema" typo in usage documentation.

docs/source/manpage.rst

@@ -72,7 +72,7 @@ All options available as of Flake8 3.1.0::
     --exit-zero           Exit with status code "0" even if there are errors.
     --install-hook=INSTALL_HOOK
                           Install a hook that is run prior to a commit for the
-                          supported version control systema.
+                          supported version control system.
     -j JOBS, --jobs=JOBS  Number of subprocesses to use to run checks in
                           parallel. This is ignored on Windows. The default,
                           "auto", will auto-detect the number of processors

src/flake8/main/options.py

@@ -158,7 +158,7 @@ def register_default_options(option_manager):
         '--install-hook', action='callback', type='choice',
         choices=vcs.choices(), callback=vcs.install,
         help='Install a hook that is run prior to a commit for the supported '
-             'version control systema.'
+             'version control system.'
     )
 
     add_option(