docs: fix Parallels Image "dirty bitmap" section

docs: fix Parallels Image "dirty bitmap" section

Original specification says that l1 table size if 64 * l1_size, which
is obviously wrong. The size of the l1 entry is 64 _bits_, not bytes.
Thus 64 is to be replaces with 8 as specification says about bytes.

There is also minor tweak, field name is renamed from l1 to l1_table,
which matches with the later text.

Signed-off-by: Denis V. Lunev <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-id: 20210128171313.2210947[email protected]
CC: Stefan Hajnoczi <[email protected]>
CC: Vladimir Sementsov-Ogievskiy <[email protected]>
[Replace the original commit message "docs: fix mistake in dirty bitmap
feature description" as suggested by Eric Blake.
--Stefan]

Signed-off-by: Stefan Hajnoczi <[email protected]>