docs/vm: fix 'mm_count' vs 'mm_users' counter confusion

In the context of the anonymous address space lifespan description the
'mm_users' reference counter is confused with 'mm_count'.  I.e a "zombie"
mm gets released when "mm_count" becomes zero, not "mm_users".
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lkml.kernel.org/r/1597040695-32633-1-git-send-email-agordeev@linux.ibm.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/active_mm.rst

@@ -64,7 +64,7 @@ Active MM
  actually get cases where you have a address space that is _only_ used by
  lazy users. That is often a short-lived state, because once that thread
  gets scheduled away in favour of a real thread, the "zombie" mm gets
- released because "mm_users" becomes zero.
+ released because "mm_count" becomes zero.
 
  Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any
  more. "init_mm" should be considered just a "lazy context when no other