[locale.ctype.virtuals] Clarify do_widen parameter

Author: jensmaurer

source/locales.tex

@@ -1227,7 +1227,7 @@
 from a \tcode{char} value or sequence of \tcode{char} values
 to the corresponding \tcode{charT} value or values.
 \begin{footnote}
-The char argument of \tcode{do_widen} is intended to
+The parameter \tcode{c} of \tcode{do_widen} is intended to
 accept values derived from \grammarterm{character-literal}s
 for conversion to the locale's encoding.
 \end{footnote}