[format.string.std] Fix broken markup (missing backslash)

Author: tkoeppe

source/utilities.tex

@@ -14859,7 +14859,7 @@
 \end{ncbnf}
 
 \pnum
-Field widths are specified in defnadj{field width}{units};
+Field widths are specified in \defnadj{field width}{units};
 the number of column positions required to display a sequence of
 characters in a terminal.
 The \defnadj{minimum}{field width}