[c.mb.wcs] Add \tcode for n (#7572)

Author: Andreas-Krug

source/text.tex

@@ -13413,7 +13413,7 @@
 that corresponds to the \unicode{0000}{null} Unicode character
 (which is the value stored).
 \item between \tcode{1} and \tcode{n} (inclusive),
-if the next n or fewer bytes complete a valid multibyte character
+if the next \tcode{n} or fewer bytes complete a valid multibyte character
 (whose first (or only) code unit is stored);
 the value returned is the number of bytes that complete the multibyte character.
 \item \tcode{(size_t)(-3)}, if the next code unit