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Fix the instructions for resetting to the BL3-1 entrypoint in the
user guide. The BL3-1 and BL3-2 image locations changed in the fix
to ARM-software/tf-issues#117 (commit a1b6db6c).

Fixes ARM-software/tf-issues#237

Change-Id: I764eb17c66034511efb984c0e7cfda29bd99198f