[basic.fundamental] Change placeholder to math font (#8199)

Author: eisenwave

source/basic.tex

@@ -5318,7 +5318,7 @@
 \defnadjx{signed integer}{types}{type}.
 The range of representable values for a signed integer type is
 $-2^{N-1}$ to $2^{N-1}-1$ (inclusive),
-where \placeholder{N} is called the \defn{width} of the type.
+where $N$ is called the \defn{width} of the type.
 \indextext{integral type!implementation-defined \tcode{sizeof}}%
 \begin{note}
 Plain \tcode{int}s are intended to have