Remove allDifferentiableVariables usage from README. (tensorflow#420)

Author: dan-zheng

README.md

@@ -57,7 +57,7 @@ for _ in 0..<1000 {
         print("Loss: \(loss)")
         return loss
     }
-    optimizer.update(&classifier.allDifferentiableVariables, along: 𝛁model)
+    optimizer.update(&classifier, along: 𝛁model)
 }
 ```
 
@@ -69,7 +69,7 @@ for _ in 0..<1000 {
     let (loss, 𝛁ŷ) = ŷ.valueWithGradient { ŷ in softmaxCrossEntropy(logits: ŷ, labels: y) }
     print("Model output: \(ŷ), Loss: \(loss)")
     let 𝛁model = backprop(𝛁ŷ)
-    optimizer.update(&classifier.allDifferentiableVariables, along: 𝛁model)
+    optimizer.update(&classifier, along: 𝛁model)
 }
 ```