doc/install.html: fix erroneous HTML annotation align=middle is invalid; use align=center
doc/install.html: fix erroneous HTML annotation
align=middle is invalid; use align=center

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/106910044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang