[simd.mask.namedconv] Use math expression for precondition

Author: mattkretz

source/numerics.tex

@@ -19354,7 +19354,7 @@
 \pnum
 \expects
 \begin{itemize}
-\item \tcode{size() <= $N$} is \tcode{true}, or
+\item \tcode{size()} $\le N$, or
 \item for all $i$ in the range \range{$N$}{size()}, \tcode{operator[]($i$)} returns \tcode{false}.
 \end{itemize}