Rename header_offset and header_interval options

Header doesn't really express that this has to do with io verification.
So rename the options to verify_offset and verify_interval.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 8cb849c..2fbbc1d 100644
--- a/HOWTO
+++ b/HOWTO
@@ -603,11 +603,11 @@
 		fast IO where the red-black tree sorting CPU time becomes
 		significant.
 
-header_offset=siint	Swap the verification header with data somewhere else
+verify_offset=siint	Swap the verification header with data somewhere else
 			in the block before writing. Its swapped back before
 			verifying.
 
-header_interval=siint	Write the verification header at a finer granularity
+verify_interval=siint	Write the verification header at a finer granularity
 			than the blocksize. It will be written for chunks the
 			size of header_interval. blocksize should divide this
 			evenly.