doc: Fix some errors in ras.rst

doc: Fix some errors in ras.rst

Make some miscellaneous fixes to the first paragraph of "ECC memory":
 - Change the incorrect "74 bits" to "72 bits".
 - Change "mentioned on" to "mentioned in".
 - Remove the extra "extra".
 - Rephrase some sentences as suggested by Matthew Wilcox.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20200506162217.16633-1-longman@redhat.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>