docs/manual/contribute.txt: fix typo

"who sponsored who sponsored" -> "who sponsored"

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/contribute.adoc

@@ -259,7 +259,7 @@ separated from it by a plus `+` sign. E.g.:
 +
 `Your-Name Your-Surname <your-name.your-surname+companyname@mail.com>`
 
-* for an individual who sponsored who sponsored the submitted work, use
+* for an individual who sponsored the submitted work, use
   their name and surname:
 +
 `Your-Name Your-Surname <your-name.your-surname+their-name.their-surname@mail.com>`