[vector.bool] Fix wording - change "variable template" to "expression"

Author: burblebee

source/containers.tex

@@ -9293,9 +9293,9 @@
 
 \begin{itemdescr}
 \pnum
-The variable template
+The expression
 \tcode{\exposid{is-vector-bool-reference}<T>} is \tcode{true}
-if \tcode{T} denotes the type \tcode{vector<bool, Alloc>::reference}
+if \tcode{T} denotes the type \tcode{vector<bool, Alloc>::\linebreak{}reference}
 for some type \tcode{Alloc} and
 \tcode{vector<bool, Alloc>} is not a program-defined specialization.
 \end{itemdescr}