Fix warnings in Bazel Swift example library

* Require callers to use argument labels so that the call site looks the same in Swift 2 and 3.

--
PiperOrigin-RevId: 144484042
MOS_MIGRATED_REVID=144484042
diff --git a/examples/swift/foo.swift b/examples/swift/foo.swift
index 1dd35c4..4f80dad 100644
--- a/examples/swift/foo.swift
+++ b/examples/swift/foo.swift
@@ -4,6 +4,6 @@
   public init() {}
 
   public func multiply() -> Int {
-    return Multiplier().multiply(Constants.x, Constants.y)
+    return Multiplier().multiply(a: Constants.x, b: Constants.y)
   }
 }