Update 2.4.txt

Fixed typos in the 2.4 release notes

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1260 copy edits to remove doubled "the"s
Update subscriber edition link to working non-ssl page

Previous URL which doesn't render properly on my machine: 
https://www.10gen.com/products/mongodb-enterprise
New URL which looks much better:
http://www.10gen.com/products/mongodb-enterprise

Signed-off-by: Sam Kleinman <samk@10gen.com>