update comment of GetNextFile

Summary: Closes https://github.com/facebook/rocksdb/pull/2377

Differential Revision: D5141274

Pulled By: lightmark

fbshipit-source-id: c237a285b73ad93488c080ea80c71a29a17f1be0

db/version_set.cc

@@ -136,8 +136,8 @@ class FilePicker {
 
         // Do key range filtering of files or/and fractional cascading if:
         // (1) not all the files are in level 0, or
-        // (2) there are more than 3 Level 0 files
-        // If there are only 3 or less level 0 files in the system, we skip
+        // (2) there are more than 3 current level files
+        // If there are only 3 or less current level files in the system, we skip
         // the key range filtering. In this case, more likely, the system is
         // highly tuned to minimize number of tables queried by each query,
         // so it is unlikely that key range filtering is more efficient than