[cpp.import] Change "directive" to "definition" in "active macro directive" (#7292)

The term 'active macro directive' is defined in p6, but never used.
Meanwhile, there are multiple uses of 'active macro definition' but
that term is never defined, including in the very sentence following
the definition of the unused term, and in other clauses that
cross-reference to this clause for their definition.

Author: AlisdairM

source/preprocessor.tex

@@ -912,8 +912,8 @@
 \end{itemize}
 
 \pnum
-\indextext{active macro directive|see{macro, active}}%
-A macro directive is \defnx{active}{macro!active} at a source location
+\indextext{active macro definition|see{macro, active}}%
+A macro definition is \defnx{active}{macro!active} at a source location
 if it has a point of definition in that translation unit preceding the location,
 and does not have a point of undefinition in that translation unit preceding
 the location.