docs/howto/*: re-generate

The script determines that it does not need to do anything if the
`docs/howto/*.html` in question exists already. But we have to fix the
`.txt` extension, which is now the `.adoc` extension.

So, after `git rm external/docs/content/docs/howto/*.html`, I simply
re-built them via:

	REBUILD_DOC=v2.50.0 RERUN=1 \
	bundle exec ruby script/update-docs.rb git-worktree en

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

external/docs/content/docs/howto/maintain-git.html

@@ -1,5 +1,5 @@
 ---
 ### DO NOT EDIT! Generated by script/update-docs.rb
 
-redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/maintain-git.txt
+redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/maintain-git.adoc
 ---

external/docs/content/docs/howto/revert-a-faulty-merge.html

@@ -1,5 +1,5 @@
 ---
 ### DO NOT EDIT! Generated by script/update-docs.rb
 
-redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/revert-a-faulty-merge.txt
+redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/revert-a-faulty-merge.adoc
 ---

external/docs/content/docs/howto/setup-git-server-over-http.html

@@ -1,5 +1,5 @@
 ---
 ### DO NOT EDIT! Generated by script/update-docs.rb
 
-redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/setup-git-server-over-http.txt
+redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/setup-git-server-over-http.adoc
 ---

external/docs/content/docs/howto/update-hook-example.html

@@ -1,5 +1,5 @@
 ---
 ### DO NOT EDIT! Generated by script/update-docs.rb
 
-redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/update-hook-example.txt
+redirect_to: https://github.com/git/git/blob/HEAD/Documentation/howto/update-hook-example.adoc
 ---