Fix reset to BL3-1 instructions in user guide

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 a1b6db6).

Fixes ARM-software/tf-issues#237

Change-Id: I764eb17c66034511efb984c0e7cfda29bd99198f
Merge pull request #186 from danh-arm/dh/fix-reset-to-bl31

Fix reset to BL3-1 instructions in user guide