Documentation/maintainer-tip: Fix typos

Fix typos in documentation: a -> an.

Signed-off-by: Andrew Kreimer <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/process/maintainer-tip.rst

···
 ---------------------
 
 The tip tree is a collection of several subsystems and areas of
-development. The tip tree is both a direct development tree and a
+development. The tip tree is both a direct development tree and an
 aggregation tree for several sub-maintainer trees. The tip tree gitweb URL
 is: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
 
···
 prefix. 'git log path/to/file' should give you a reasonable hint in most
 cases.
 
-The condensed patch description in the subject line should start with a
+The condensed patch description in the subject line should start with an
 uppercase letter and should be written in imperative tone.