doc: fix undo data filename (s/undo???.dat/rev???.dat/)

Author: theStack

src/validation.h

@@ -402,7 +402,7 @@ class BlockManager
     void FindFilesToPruneManual(std::set<int>& setFilesToPrune, int nManualPruneHeight, int chain_tip_height);
 
     /**
-     * Prune block and undo files (blk???.dat and undo???.dat) so that the disk space used is less than a user-defined target.
+     * Prune block and undo files (blk???.dat and rev???.dat) so that the disk space used is less than a user-defined target.
      * The user sets the target (in MB) on the command line or in config file.  This will be run on startup and whenever new
      * space is allocated in a block or undo file, staying below the target. Changing back to unpruned requires a reindex
      * (which in this case means the blockchain must be re-downloaded.)