firmware-design: Fix typo in ToC header flags specification

Fixes ARM-software/tf-issues#463

Change-Id: I73e0c5fbd87004953df8b1fa19319ad562ecc867
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

docs/firmware-design.md

@@ -1752,7 +1752,7 @@ The ToC header has the following fields:
     `name`: The name of the ToC. This is currently used to validate the header.
     `serial_number`: A non-zero number provided by the creation tool
     `flags`: Flags associated with this data.
-        Bits 0-13: Reserved
+        Bits 0-31: Reserved
         Bits 32-47: Platform defined
         Bits 48-63: Reserved