[preset_manager/tutorial.md] misunderstanding reference in the double pendulum example section

[osrf-migration]

Original report (archived issue) by Alessandro Tondo (Bitbucket: alextoind).

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In the double pendulum example have been chosen 3 different reference frames:

• text: it is said that "the pendulum to swing back and forth in the XZ plane" (i.e. revolute_joint = [0 1 0]);
• image: it is shown that the pendulum oscillate in the XY plane (i.e. revolute_joint = [0 0 1]);
• code: the revolute joints have <xyz>1 0 0</xyz> property (i.e. revolute_joint = [1 0 0]).

Anyway, I think that the purpose of the example is achieved the same, but there is a bit of ambiguity.