doc: put text into header of 1st column in table to generate page correctly
>From the table without header text of 1st column, docutils generates
the table with fully empty header row.
$ hg init debugrevlog
$ cd debugrevlog
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debugrevlog -m
format : 1
flags : inline
revisions : 1
merges : 0 ( 0.00%)
normal : 1 (100.00%)
revisions : 1
full : 1 (100.00%)
deltas : 0 ( 0.00%)
revision size : 44
full : 44 (100.00%)
deltas : 0 ( 0.00%)
avg chain length : 0
compression ratio : 0
uncompressed data size (min/max/avg) : 43 / 43 / 43
full revision size (min/max/avg) : 44 / 44 / 44
delta size (min/max/avg) : 0 / 0 / 0