Remove the following redundant sentence from the user guide, which
implies the user should use the TF version from the Linaro release,
which was not the intention:

"However, the rest of this document assumes that you got the
Trusted Firmware as part of the Linaro release."

Also, tidied up the grammar in this section.

Change-Id: I5dae0b68d3683e2a85a7b3c6a31222182a66f6c8