Eliminate duplication of MethodDecl nonterminal by renaming

the one associated with interface types.

R=gri
DELTA=3  (0 added, 0 deleted, 3 changed)
OCL=15092
CL=15096

doc/go_spec.txt

@@ -1090,9 +1090,9 @@ Interface types
 
 An interface type denotes a set of methods.
 
-	InterfaceType = "interface" "{" [ MethodDeclList [ ";" ] ] "}" .
-	MethodDeclList = MethodDecl { ";" MethodDecl } .
-	MethodDecl = identifier FunctionType .
+	InterfaceType = "interface" "{" [ MethodList [ ";" ] ] "}" .
+	MethodList = Method { ";" Method } .
+	Method = identifier FunctionType .
 
 	// A basic file interface.
 	type File interface {