[expr.prim.lambda.capture] Add missing \keyword

This fixes a misapplication of CWG1249, which requested "mutable" to
be code, not text.

Author: tkoeppe

source/expressions.tex

@@ -2515,7 +2515,7 @@
 \item If \tcode{m1} captures the entity by copy,
 \tcode{m2} captures the corresponding
 non-static data member of \tcode{m1}'s closure type;
-if \tcode{m1} is not mutable, the non-static data member is considered to be const-qualified.
+if \tcode{m1} is not \keyword{mutable}, the non-static data member is considered to be const-qualified.
 \item If \tcode{m1} captures the entity by reference,
 \tcode{m2} captures the same
 entity captured by \tcode{m1}.