[class.friend] Fix a mistakenly monospaced "friend declaration" (#6736)

Author: Quuxplusone

source/classes.tex

@@ -4850,7 +4850,7 @@
 a \grammarterm{template-declaration}\iref{temp.pre,temp.friend}.
 \end{note}
 If the
-type specifier in a \keyword{friend} declaration designates a (possibly
+type specifier in a friend declaration designates a (possibly
 cv-qualified) class type, that class is declared as a friend; otherwise, the
 friend declaration is ignored.
 \begin{example}