doc: Fix variables names in TBBR CoT documentation
In commit 516beb5 ("TBB: apply TBBR naming
convention to certificates and extensions"), some of the variables used in the
TBBR chain of trust got renamed but the documentation did not get properly
updated everywhere to reflect these changes.
Change-Id: Ie8e2146882c2d3538c5b8c968d1bdaf5ea2a6e53
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>