nfsd: fix Documentation typo

Caught by Benny, thanks!
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

Documentation/filesystems/nfs41-server.txt

@@ -41,7 +41,7 @@ interoperability problems with future clients.  Known issues:
 	  conformant with the spec (for example, we don't use kerberos
 	  on the backchannel correctly).
 	- no trunking support: no clients currently take advantage of
-	  trunking, but this is a mandatory failure, and its use is
+	  trunking, but this is a mandatory feature, and its use is
 	  recommended to clients in a number of places.  (E.g. to ensure
 	  timely renewal in case an existing connection's retry timeouts
 	  have gotten too long; see section 8.3 of the draft.)