[lex.pptoken] Consistent use of preprocessing vs processing

There are three cases here all doing the same thing.  Two
refer to preprocessing a directive, while the first refers
to processing without the pre.

Author: AlisdairM

source/lex.tex

@@ -571,7 +571,7 @@
 
 \pnum
 The \grammarterm{import-keyword} is produced
-by processing an \keyword{import} directive\iref{cpp.import},
+by preprocessing an \keyword{import} directive\iref{cpp.import},
 the \grammarterm{module-keyword} is produced
 by preprocessing a \keyword{module} directive\iref{cpp.module}, and
 the \grammarterm{export-keyword} is produced