docs/manual: 'Fixes' tag needs a colon

Apparently, patchwork only recognizes the 'Fixes' tag if it is followed
by a colon. So make sure the manual documents it as such.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/contribute.txt

@@ -73,7 +73,7 @@ basically two things that can be done:
     In the commit message of a patch fixing an autobuild failure, add a
     reference to the build result directory, as follows:
 ---------------------
-Fixes http://autobuild.buildroot.org/results/51000a9d4656afe9e0ea6f07b9f8ed374c2e4069
+Fixes: http://autobuild.buildroot.org/results/51000a9d4656afe9e0ea6f07b9f8ed374c2e4069
 ---------------------
 
 === Reviewing and testing patches