doc/install.html: fix duplicate id= tag
doc/install.html: fix erroneous HTML
doc: link to new downloads page

https://code.google.com/p/go/source/browse/doc/install.html?r=951cc5f6d52ff1dc2bb9d8e6b2fb98fe1699a34f
Merge pull request #60 from t-yuki/master

doc/install.html: update to tip version 951cc5f6d52ff1dc2bb9d8e6b2fb98fe1699a34f