Fix getRemoteKeysForTarget() method name in comment. (#1020)

While porting I noticed this was slightly wrong. targetContainsDocument() is the method in WatchChangeAggregator. The SyncEngine method I meant to reference is getRemoteKeysForTarget().

Author: mikelehen

packages/firestore/src/core/sync_engine.ts

@@ -99,7 +99,7 @@ class LimboResolution {
 
   /**
    * Set to true once we've received a document. This is used in
-   * targetContainsDocument() and ultimately used by WatchChangeAggregator to
+   * getRemoteKeysForTarget() and ultimately used by WatchChangeAggregator to
    * decide whether it needs to manufacture a delete event for the target once
    * the target is CURRENT.
    */