_capabilities documentation fix (#4434)
_capabilities documentation fix (#4434)

$capabilityFunction should return array, otherwise on second test run you got:
```
[PHPUnit_Framework_Exception] Argument 1 passed to Facebook\WebDriver\Remote\DesiredCapabilities::__construct() must be of the type array, object given
```
Codeception/base,Codeception/base,Naktibalda/Codeception,Codeception/Codeception,Naktibalda/Codeception,tasselchof/Codeception,tasselchof/Codeception,Codeception/Codeception,Codeception/base,Naktibalda/Codeception,tasselchof/Codeception