[exec.cmplsig] add missing \exposid

Author: ericniebler

source/exec.tex

@@ -6107,7 +6107,7 @@
                     @\itcorr[1]\exposid{META-APPLY}@(Tuple, Ts@$_{m-1}$@...))
 \end{codeblock}
 where $m$ is the size of the pack \tcode{TagFns} and
-\tcode{META-APPLY(T, As...)} is equivalent to:
+\tcode{\exposid{META-APPLY}(T, As...)} is equivalent to:
 \begin{codeblock}
 typename @\exposid{indirect-meta-apply}@<@\exposid{always-true}@<As...>>::template @\exposid{meta-apply}@<T, As...>
 \end{codeblock}