doc/install.html: fix erroneous HTML annotation

align=middle is invalid; use align=center

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/106910044

doc/install.html

@@ -39,9 +39,9 @@ proceeding. If your OS or architecture is not on the list, it's possible that
 
 <table class="codetable" frame="border" summary="requirements">
 <tr>
-<th align="middle">Operating system</th>
-<th align="middle">Architectures</th>
-<th align="middle">Notes</th>
+<th align="center">Operating system</th>
+<th align="center">Architectures</th>
+<th align="center">Notes</th>
 </tr>
 <tr><td colspan="3"><hr></td></tr>
 <tr><td>FreeBSD 8 or later</td> <td>amd64, 386, arm</td> <td>Debian GNU/kFreeBSD not supported; FreeBSD/ARM needs FreeBSD 10 or later</td></tr>