[concept.copyconstructible] Avoid "possibly \tcode{const}"

A small rewrite avoids the phrase "possibly \tcode{const}",
which we would like to remove entirely in favour of just
"possibly const".

Author: tkoeppe

source/concepts.tex

@@ -833,7 +833,7 @@
 \begin{itemdescr}
 \pnum
 If \tcode{T} is an object type, then let \tcode{v} be an lvalue of type
-(possibly \keyword{const}) \tcode{T} or an rvalue of type \tcode{const T}.
+\tcode{T} or \tcode{\keyword{const} T} or an rvalue of type \tcode{const T}.
 \tcode{T} models \libconcept{copy_constructible} only if
 
 \begin{itemize}