Doxyfile: Fix typo in comment (found by codespell)

Signed-off-by: Stefan Weil <sw@weilnetz.de>

doc/Doxyfile

@@ -1657,7 +1657,7 @@ EXTRA_PACKAGES         =
 # following commands have a special meaning inside the header: $title,
 # $datetime, $date, $doxygenversion, $projectname, $projectnumber,
 # $projectbrief, $projectlogo. Doxygen will replace $title with the empy string,
-# for the replacement values of the other commands the user is refered to
+# for the replacement values of the other commands the user is referred to
 # HTML_HEADER.
 # This tag requires that the tag GENERATE_LATEX is set to YES.