Commit 290e563166 changed the flag's name from dontcache
to nocache. This patch fixes the man page.
This is a content change.
Reviewed by: kib (earlier version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D58181
Fixes: 4830670a3f ("chflags.1: Document the new UF_DONTCACHE flag")