Doc: Fix typos in ip-sysctl.txt about rp_filter.

First fix a typo in Stephens patch ;-)
Signed-off-by: NJesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -711,8 +711,8 @@ rp_filter - INTEGER
 	    the packet check will fail.
 
         Current recommended practice in RFC3704 is to enable strict mode
-	to prevent IP spoofin from DDos attacks. If using asymmetric routing
-        or other complicated routing,t hen loose mode is recommended.
+	to prevent IP spoofing from DDos attacks. If using asymmetric routing
+        or other complicated routing, then loose mode is recommended.
 
 	conf/all/rp_filter must also be set to non-zero to do source validation
 	on the interface