Fix reftex-citation docstring (bug#60710)

* lisp/textmodes/reftex-cite.el (reftex-citation): Fix spelling.

lisp/textmodes/reftex-cite.el

@@ -636,7 +636,7 @@ command, it will add another key, ignoring the value of
 
 The regular expression uses an expanded syntax: && is interpreted as `and'.
 Thus, `aaaa&&bbb' matches entries which contain both `aaaa' and `bbb'.
-While entering the regexp, completion on knows citation keys is possible.
+While entering the regexp, completion on known citation keys is possible.
 `=' is a good regular expression to match all entries in all files."
   (interactive)