DOC: don't reuse names in introductory example (keras-team#12721)

* don't reuse names in introductory example

I would try to be as explicit as possible in the first example of the functional API

* Update functional-api-guide.md

rename symbolic tensors to output

docs/templates/getting-started/functional-api-guide.md

@@ -24,9 +24,9 @@ from keras.models import Model
 inputs = Input(shape=(784,))
 
 # a layer instance is callable on a tensor, and returns a tensor
-x = Dense(64, activation='relu')(inputs)
-x = Dense(64, activation='relu')(x)
-predictions = Dense(10, activation='softmax')(x)
+output_1 = Dense(64, activation='relu')(inputs)
+output_2 = Dense(64, activation='relu')(output_1)
+predictions = Dense(10, activation='softmax')(output_2)
 
 # This creates a model that includes
 # the Input layer and three Dense layers