tools: Fixes explanation about -F option in biolatency_example.txt

-F option prints a separate histogram for each unique set of request
flags. But the explanation right after the -F option example is not
about this option.

tools/biolatency_example.txt

@@ -289,7 +289,7 @@ flags = Priority-Metadata-Write
      msecs               : count     distribution
          0 -> 1          : 9        |****************************************|
 
-These can be handled differently by the storage device, and this mode lets us
+These can be handled differently by the request flags, and this mode lets us
 examine their performance in isolation.