[expr.prim.splice] Add missing maths font for variable "C".

Misapplication of P3598R0 in CWG Motion 8.

Addresses editorial review committee feedback.

Author: tkoeppe

source/expressions.tex

@@ -3615,7 +3615,7 @@
 The expression $E$ is an lvalue referring to the object or function $X$
 associated with or referenced by $S$, and
 is a bit-field if and only if $X$ is a bit-field.
-If $E$ appears in the predicate of a contract assertion C\iref{basic.contract}
+If $E$ appears in the predicate of a contract assertion $C$\iref{basic.contract}
 and $S$ is
 \begin{itemize}
 \item