Documentation: ext4: fix cell spacing of table heading on blockmap table

Commit 3103084a ("ext4, doc: remove unnecessary escaping") removes
redundant underscore escaping, however the cell spacing in heading row of
blockmap table became not aligned anymore, hence triggers malformed table
warning:

Documentation/filesystems/ext4/blockmap.rst:3: WARNING: Malformed table.

+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| i.i_block Offset   | Where It Points                                                                                                                                                                                                              |
<snipped>...

The warning caused the table not being loaded.

Realign the heading row cell by adding missing space at the first cell
to fix the warning.

Fixes: 3103084a

 ("ext4, doc: remove unnecessary escaping")
Cc: stable@kernel.org
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Wang Jianjian <wangjianjian3@huawei.com>
Cc: linux-ext4@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220619072938.7334-1-bagasdotme@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>