[dcl.enum] use indefinite article before "fixed underlying type"

Author: jwakely

source/declarations.tex

@@ -2028,7 +2028,7 @@
 An \grammarterm{opaque-enum-declaration} is either a redeclaration
 of an enumeration in the current scope or a declaration of a new enumeration.
 \begin{note} An enumeration declared by an
-\grammarterm{opaque-enum-declaration} has fixed underlying type and is a
+\grammarterm{opaque-enum-declaration} has a fixed underlying type and is a
 complete type. The list of enumerators can be provided in a later redeclaration
 with an \grammarterm{enum-specifier}. \end{note} A scoped enumeration
 shall not be later redeclared as unscoped or with a different underlying type.