Minor formatting fixes to CONTRIBUTING.md
Minor formatting fixes to CONTRIBUTING.md

Fix inconsistent list formatting in CONTRIBUTING.md.

Signed-off-by: Dan Handley <f921e9c08311ccef2e6e7757de21685f66c391dc@arm.com>

NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls