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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/ras.rst

@@ -156,11 +156,11 @@ the labels provided by the BIOS won't match the real ones.
 ECC memory
 ----------
 
-As mentioned on the previous section, ECC memory has extra bits to be
-used for error correction. So, on 64 bit systems, a memory module
-has 64 bits of *data width*, and 74 bits of *total width*. So, there are
-8 bits extra bits to be used for the error detection and correction
-mechanisms. Those extra bits are called *syndrome*\ [#f1]_\ [#f2]_.
+As mentioned in the previous section, ECC memory has extra bits to be
+used for error correction. In the above example, a memory module has
+64 bits of *data width*, and 72 bits of *total width*.  The extra 8
+bits which are used for the error detection and correction mechanisms
+are referred to as the *syndrome*\ [#f1]_\ [#f2]_.
 
 So, when the cpu requests the memory controller to write a word with
 *data width*, the memory controller calculates the *syndrome* in real time,