Fix a typo in Doxyfile

doc/Doxyfile

@@ -1113,7 +1113,7 @@ HTML_STYLESHEET        =
 # cascading style sheets that are included after the standard style sheets
 # created by doxygen. Using this option one can overrule certain style aspects.
 # This is preferred over using HTML_STYLESHEET since it does not replace the
-# standard style sheet and is therefor more robust against future updates.
+# standard style sheet and is therefore more robust against future updates.
 # Doxygen will copy the style sheet files to the output directory.
 # Note: The order of the extra stylesheet files is of importance (e.g. the last
 # stylesheet in the list overrules the setting of the previous ones in the