Remove bad reference to no-arg increment

Change-Id: I307167514d07fafaf9f6fa8ad1cea46a7867f6e0

Author: samtstern

firestore/main/index.js

@@ -280,7 +280,6 @@ function updateDocumentIncrement(db) {
   var washingtonRef = db.collection('cities').doc('DC');
 
   // Atomically increment the population of the city by 50.
-  // Note that increment() with no arguments increments by 1.
   var popIncrement = washingtonRef.update({
     population: admin.firestore.FieldValue.increment(50)
   });