[library] Fix index entry

Author: tkoeppe

source/lib-intro.tex

@@ -2664,7 +2664,7 @@
 Clauses~\ref{\firstlibchapter} through~\ref{\lastlibchapter} and Annex~\ref{depr} is a
 \tcode{friend}
 of another class in the \Cpp standard library.
-\indextext{specifier!\tcode{friend}}
+\indextext{specifier!\idxcode{friend}}
 
 \rSec3[derivation]{Derived classes}