Documentation: e1000: Use correct heading adornment

Recently documentation file was converted to rst.  The document title
has the incorrect heading adornment.  From kernel docs:

	* Please stick to this order of heading adornments:

	  1. ``=`` with overline for document title::

	       ==============
	       Document title
	       ==============

Add  overline heading adornment to document title.

Fixes commit (228046e7 Documentation: e1000: Update kernel documentation)

CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/e1000.rst

+===========================================================
 Linux* Base Driver for Intel(R) Ethernet Network Connection
 ===========================================================