[except.spec] Excise undefined term 'instantiation directive'. (#4312)

Author: jensmaurer

source/exceptions.tex

@@ -731,7 +731,7 @@
 a \grammarterm{noexcept-specifier} may be specified,
 but is not required.
 If a \grammarterm{noexcept-specifier} is specified
-in an explicit instantiation directive,
+in an explicit instantiation,
 the exception specification shall be the same as
 the exception specification of all other declarations of that function.
 A diagnostic is required only if the