doc: crypto: Match documentation file names to documentation titles

Rename the documentation file names for cc3xx to match their
used names in the documentation.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>

Author: joerchan

crypto/README.rst

@@ -8,8 +8,8 @@ Crypto Libraries
    :caption: Subpages:
 
    doc/nrf_cc310_bl
-   doc/nrf_cc310_platform
-   doc/nrf_cc310_mbedcrypto
+   doc/nrf_cc3xx_platform
+   doc/nrf_cc3xx_mbedcrypto
    doc/nrf_oberon
    doc/CHANGELOG_cc310_bl
    doc/CHANGELOG_cc3xx_platform