Documentation: Spell "blob" in lowercase

It's not an acronym (outside of database software).

Change-Id: I529561e4fc9889be7f9d6bd6d5f9a876e2007671
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/28827
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/northbridge/intel/sandybridge/nri_features.md

@@ -8,7 +8,7 @@
     * There might be errors to fix.
     * Position in romstage doesn't matter.
 2.  mrc.bin raminit
-    * Closed Source (aka BLOB)
+    * Closed Source (aka blob)
     * No known errors.
     * Needs to be placed at fixed offset in romstage.