integrations/redmine: Clean up a bit of grammar in the docs.
integrations/redmine: Clean up a bit of grammar in the docs.

It's kind of awkward that this takes the scheme-relative URL -- either
the full URL, or maybe just the host, would make more sense.  But on a
bit of study of the plugin's source, I can't convince myself that it
won't break if given the full URL with the scheme.  (See a code
comment thread in #7116.)  So leave the substance as is, pending the
plugin itself getting some cleanup, but do fix the sentence's English
grammar since we're looking at it.

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