doc: Correct text of error injection in Fault Management

Step 7 in fault management, says "Inject the same fault into the
System FMU again", but the next instruction doesn't inject the
same fault as the previous. It should say "Inject another
Lockstep error into the System FMU".

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>