Documentation/eth/intel: Remove references to SourceForge

The out-of-tree driver is hosted on SourceForge, as this does not apply
to the kernel driver remove references to it. Also do some minor
formatting changes around this section.

Suggested-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>