[class.access.general] Fix improper \keyword{private} (#6532)

Author: eisenwave

source/classes.tex

@@ -4229,9 +4229,7 @@
 \indextext{access control!default}%
 Members of a class defined with the keyword
 \keyword{class}
-are
-\keyword{private}
-by default.
+are private by default.
 Members of a class defined with the keywords
 \keyword{struct} or \keyword{union}
 are public by default.