Replace dead urls with active urls for Mutt

This patch replace stale/dead urls with active urls for Mutt.
Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/email-clients.rst

@@ -237,9 +237,9 @@ using Mutt to send patches through Gmail::
 
 The Mutt docs have lots more information:
 
-    http://dev.mutt.org/trac/wiki/UseCases/Gmail
+    https://gitlab.com/muttmua/mutt/-/wikis/UseCases/Gmail
 
-    http://dev.mutt.org/doc/manual.html
+    http://www.mutt.org/doc/manual/
 
 Pine (TUI)
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