docs/subset-A.html: fix markup

docs/subset-A.html: fix markup

fixes tidy warnings:

line 11 column 1 - Warning: <center> isn't allowed in <h1> elements
line 10 column 1 - Info: <h1> previously mentioned
line 11 column 34 - Warning: discarding unexpected </center>
line 14 column 1 - Warning: <center> isn't allowed in <h2> elements
line 13 column 1 - Info: <h2> previously mentioned
line 13 column 1 - Warning: missing </h2> before <h3>
line 18 column 1 - Warning: discarding unexpected </center>
line 19 column 1 - Warning: discarding unexpected </h2>

Signed-off-by: Brian Paul <brianp@vmware.com>