The link takes you to the SQL Server 2012 entry, it just doesn't tell you that very well. Look on the left hand navigation hierarchy, it shows 'Books Online for SQL Server 2012'.

The SQL Server 2008 R2 reference is located here (notice the versioning in the parentheses):

http://msdn.microsoft.com/en-us/library/ms175941(v=sql.105).aspx

The SQL Server 2008 reference is located here:

http://msdn.microsoft.com/en-us/library/ms175941(v=sql.100).aspx

Unfortunately when you navigate to the above links, it doesn't give the opportunity to get back to the 2012 version.