Fix documentation for CnP bit

The CnP bit documentation in the Firmware Design Guide incorrectly
used the term "Page Entries" instead of "Processing Elements".
Fix that.

Change-Id: Ie44ee99c281b7b1a9ad90fba2c7d109f12425507
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

docs/firmware-design.rst

@@ -2574,9 +2574,9 @@ This Architecture Extension is targeted when ``ARM_ARCH_MAJOR`` == 8 and
 ``ARM_ARCH_MINOR`` >= 2.
 
 -  The Common not Private (CnP) bit is enabled to indicate that multiple
-   Page Entries in the same Inner Shareable domain use the same translation
-   table entries for a given stage of translation for a particular translation
-   regime.
+   Processing Elements in the same Inner Shareable domain use the same
+   translation table entries for a given stage of translation for a particular
+   translation regime.
 
 ARMv7
 ~~~~~