Update Git maintainer's guide link

Author: psibi

book/05-distributed-git/sections/maintaining.asc

@@ -364,7 +364,7 @@ When a topic branch has finally been merged into `master`, it's removed from the
 The Git project also has a `maint` branch that is forked off from the last release to provide backported patches in case a maintenance release is required.
 Thus, when you clone the Git repository, you have four branches that you can check out to evaluate the project in different stages of development, depending on how cutting edge you want to be or how you want to contribute; and the maintainer has a structured workflow to help them vet new contributions.
 The Git project's workflow is specialized.
-To clearly understand this you could check out the https://github.com/git/git/blob/master/Documentation/howto/maintain-git.txt[Git Maintainer's guide^].
+To clearly understand this you could check out the https://github.com/git/git/blob/master/Documentation/howto/maintain-git.adoc[Git Maintainer's guide^].
 
 [[_rebase_cherry_pick]]
 ===== Rebasing and Cherry-Picking Workflows