corrected small typos

doc/api/dgram.markdown

@@ -192,10 +192,10 @@ interfaces.
 
 ### dgram.dropMembership(multicastAddress, [multicastInterface])
 
-Opposite of `dropMembership` - tells the kernel to leave a multicast group with
+Opposite of `addMembership` - tells the kernel to leave a multicast group with
 `IP_DROP_MEMBERSHIP` socket option. This is automatically called by the kernel
 when the socket is closed or process terminates, so most apps will never need to call
 this.
 
-If `multicastAddress` is not specified, the OS will try to add membership to all valid
+If `multicastAddress` is not specified, the OS will try to drop membership to all valid
 interfaces.