Documentation fix: use \p to reference a parameter

The doc of `Follow::create()` was broken due to the use of @param tag.

    /** creates the action with a set boundary,
     It will work with no boundary if @param rect is equal to Rect::ZERO.
     */

renders:

    creates the action with a set boundary, It will work with no boundary if
    Parameters
        rect	is equal to Rect::ZERO.

The 'work with no boundary if' part was meant to be followed by 'rect is equal to Rect::ZERO' part.

The orginal @param is a way to describe a parameter. To reference a parameter, use `\p` instead.

Ref: http://www.stack.nl/~dimitri/doxygen/manual/commands.html#cmdp