[class.pre] Insert missing space between words (#7754)

Author: Andreas-Krug

source/classes.tex

@@ -128,7 +128,7 @@
 
 \pnum
 Each \grammarterm{class-property-specifier} shall appear at most once
-withina single \grammarterm{class-property-specifier-seq}.
+within a single \grammarterm{class-property-specifier-seq}.
 Whenever a \grammarterm{class-key} is followed
 by a \grammarterm{class-head-name},
 the identifier \tcode{final}, \tcode{trivially_relocatable_if_eligible},