[concept.booleantestable] Use plain \exposconcept (#5657)

Author: hewillk

source/concepts.tex

@@ -981,7 +981,7 @@
 \tcode{true_type}\iref{meta.type.synop},
 \tcode{int*}, and
 \tcode{bitset<N>::reference}\iref{template.bitset}
-model \exposconceptx{boolean\--testable}{boolean-testable}.
+model \exposconcept{boolean-testable}.
 \end{example}
 
 \rSec2[concept.equalitycomparable]{Concept \cname{equality_comparable}}