[BUGFIX] Indexing of external files does not work in indexed_search

After the namespace change a wrong replace was done in
indexed_search. Replace it with the correct class.

Change-Id: I35295f4fecde241ec3004a8ee6b7dd71d5e34e93
Resolves: #47106
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22595
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn