Docs: Rename duplicate title in porting guide
Docs: Rename duplicate title in porting guide

Fix one of the two titles that ended up being the same, although both
describe different things.

Change-Id: I66ecf369643709898ee4c014659d8f85c0480643

achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware,sbranden/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public