Fix API name in a comment in db.h

Summary:
... so that people are not confused.
Closes https://github.com/facebook/rocksdb/pull/3580

Differential Revision: D7187175

Pulled By: sagar0

fbshipit-source-id: bce70093d52e38cd24c9432fd708885d7c2c013e

include/rocksdb/db.h

@@ -826,7 +826,7 @@ class DB {
 
   // This function will wait until all currently running background processes
   // finish. After it returns, no background process will be run until
-  // UnblockBackgroundWork is called
+  // ContinueBackgroundWork is called
   virtual Status PauseBackgroundWork() = 0;
   virtual Status ContinueBackgroundWork() = 0;