faq: go does not have duck typing

R=golang-dev, 0xjnml, iant, adonovan, aram
CC=golang-dev
https://golang.org/cl/6500092

doc/go_faq.html

@@ -541,7 +541,7 @@ Why doesn't Go have "implements" declarations?</h3>
 <p>
 A Go type satisfies an interface by implementing the methods of that interface,
 nothing more.  This property allows interfaces to be defined and used without
-having to modify existing code.  It enables a kind of "duck typing" that
+having to modify existing code.  It enables a kind of structural typing that
 promotes separation of concerns and improves code re-use, and makes it easier
 to build on patterns that emerge as the code develops.
 The semantics of interfaces is one of the main reasons for Go's nimble,