[temp.inst] Fix definite article

Author: Andreas-Krug

source/templates.tex

@@ -6159,7 +6159,7 @@
 \pnum
 The \grammarterm{noexcept-specifier} of a function template specialization
 is not instantiated along with the function declaration; it is instantiated
-when needed\iref{except.spec}. If such an
+when needed\iref{except.spec}. If such a
 \grammarterm{noexcept-specifier} is needed but has not yet been
 instantiated, the dependent names are looked up, the semantics constraints are
 checked, and the instantiation of any template used in the