[cpp.pre] Apply unicode markup (#8410)

Author: AlisdairM

source/preprocessor.tex

@@ -310,7 +310,7 @@
 within a preprocessing directive
 (from just after the directive-introducing token
 through just before the terminating new-line character)
-are space and horizontal-tab
+are \unicode{0020}{space} and \unicode{0009}{character tabulation}
 (including spaces that have replaced comments
 or possibly other whitespace characters
 in translation phase 3).