fixup: make underlying entity a definition

Author: jensmaurer

source/basic.tex

@@ -133,7 +133,8 @@
 template parameter,
 function parameter, or
 \grammarterm{init-capture}.
-The underlying entity of an entity is that entity unless otherwise specified.
+The \defnadj{underlying}{entity} of an entity is that entity
+unless otherwise specified.
 A name \defnx{denotes}{denote} the underlying entity of
 the entity declared by each declaration that introduces the name.
 \begin{note}