Wiki link in failsafe setup page gives 404 error

#### Issue details
If the link to the wiki page in the upper right corner of the fail safe setup screen is clicked, the web browser is sent a page that no longer exists on ardupilot.org.

This is the URL that is broken: http://ardupilot.org/copter/docs/failsafe.html

#### Version
1.3.49 Build 1.1.6410.20232
#### Platform
[ ] All
[ ] AntennaTracker
[ x ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Hexa

#### Hardware type
Cube/Pixhawk 2.1

#### Logs
n/a
Screen capture of the problem (warning: 3840x1200 px screen cap)

Search the Mission Planner repository for the broken URL `http://ardupilot.org/copter/docs/failsafe.html` and inspect the failsafe setup screen's upper-right link. Update the link to the correct destination, then verify that clicking it opens an existing page instead of returning a 404 error.

Written by the indexing model from the issue text.