[basic.lval] Replace inappropriate "\keyword" with "\tcode" (#7650)

Author: ilazaric

source/expressions.tex

@@ -321,7 +321,7 @@
 If a program invokes
 a defaulted copy/move constructor or copy/move assignment operator
 for a union of type \tcode{U} with a glvalue argument
-that does not denote an object of type \cv{}~\keyword{U} within its lifetime,
+that does not denote an object of type \cv{}~\tcode{U} within its lifetime,
 the behavior is undefined.
 \begin{note}
 In C, an entire object of structure type can be accessed, e.g., using assignment.