docs: Fix typo on pull requests guide

Signed-off-by: Marco Villegas <git@marvil07.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/maintainer/pull-requests.rst

@@ -29,7 +29,7 @@ request to.
 In order to create the pull request you must first tag the branch that you
 have just created. It is recommended that you choose a meaningful tag name,
 in a way that you and others can understand, even after some time.  A good
-practice is to include in the name an indicator of the sybsystem of origin
+practice is to include in the name an indicator of the subsystem of origin
 and the target kernel version.
 
 Greg offers the following. A pull request with miscellaneous stuff for