[basic.lookup.elab] Replace "the" with "an" (#8612)

Author: lprv

source/basic.tex

@@ -2766,7 +2766,7 @@
 \end{note}
 
 \pnum
-If the terminal name of the \grammarterm{elaborated-type-specifier}
+If the terminal name of an \grammarterm{elaborated-type-specifier}
 is a qualified name,
 lookup for it is type-only.
 If the name lookup does not find a previously declared \grammarterm{type-name},