manual/contribute.txt: fix typo

Signed-off-by: Julien Olivain <juju@cotds.org>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 0c48904f76af5d97dd3da334caecfe1fe521f072)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/contribute.txt

@@ -402,7 +402,7 @@ the problem is complex, it is OK to add more paragraphs. All paragraphs
 should be wrapped at 72 characters.
 
 A paragraph that explains the root cause of the problem. Again, more
-than on paragraph is OK.
+than one paragraph is OK.
 
 Finally, one or more paragraphs that explain how the problem is solved.
 Don't hesitate to explain complex solutions in detail.