Improve the comment on TableFactory::NewTableReader()

Summary:
`DBImpl::AddFile()` has been replaced by `DBImpl::IngestExternalFile()`.
Closes https://github.com/facebook/rocksdb/pull/3726

Differential Revision: D7646875

Pulled By: ajkr

fbshipit-source-id: 241eb7a8d88527fdc5c26b0c3f6faec3296451f8

include/rocksdb/table.h

@@ -422,7 +422,7 @@ class TableFactory {
   //     and cache the table object returned.
   // (2) SstFileReader (for SST Dump) opens the table and dump the table
   //     contents using the iterator of the table.
-  // (3) DBImpl::AddFile() calls this function to read the contents of
+  // (3) DBImpl::IngestExternalFile() calls this function to read the contents of
   //     the sst file it's attempting to add
   //
   // table_reader_options is a TableReaderOptions which contain all the