[Fortran, patch, committed] Fix copy'n'paste bug in CO_BROADCAST documentation

[Tobias Burnus]

As pointed out to me by Damian (and to him by one of his course 
attendees), the description wrongly claimed that the source-image 
argument of co_broadcast is optional. (Thanks!)

Fixed by the attached patch (Rev. 220261).

It was probably a copy'n'paste error as CO_MAX etc. take an optional 
RESULT_IMAGE argument.

Tobias
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