dgram: api style nits

Correctly mention function calls and strip trailing whitespace

doc/api/dgram.md

@@ -108,7 +108,7 @@ Tells the kernel to join a source-specific multicast channel at the given
 `IP_ADD_SOURCE_MEMBERSHIP` socket option. If the `multicastInterface` argument
 is not specified, the operating system will choose one interface and will add
 membership to it. To add membership to every available interface, call
-`addSourceSpecificMembership` multiple times, once per interface.
+`socket.addSourceSpecificMembership()` multiple times, once per interface.
 
 ### socket.address()
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