docs/manual/contribute.adoc: link to git-send-email.io

https://git-send-email.io/ is a page maintained by sourcehut which
explains how to setup git send-email on many OS's for many popular email
providers.

Signed-off-by: Marcus Hoffmann <buildroot@bubu1.eu>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/contribute.adoc

@@ -369,7 +369,7 @@ $ git send-email origin/master
 ---------------------
 
 Note that +git+ should be configured to use your mail account.
-To configure +git+, see +man git-send-email+ or google it.
+To configure +git+, see +man git-send-email+ or https://git-send-email.io/.
 
 If you do not use +git send-email+, make sure posted *patches are not
 line-wrapped*, otherwise they cannot easily be applied. In such a case,