[concept.equalitycomparable] Use \exposconcept (#5660)

Author: hewillk

source/concepts.tex

@@ -1005,7 +1005,7 @@
 \tcode{const remove_reference_t<T>} and
 \tcode{const remove_reference_t<U>} respectively.
 \tcode{T} and \tcode{U} model
-\tcode{\placeholder{weakly-equality-comparable-with}<T, U>} only if
+\tcode{\exposconcept{weakly-equality-comparable-with}<T, U>} only if
 \begin{itemize}
 \item \tcode{t == u}, \tcode{u == t}, \tcode{t != u}, and \tcode{u != t}
       have the same domain.