Update dev_guide.mvc.understanding_controller.ngdoc

[aevange]

Add the word "to" on line 66 to change the sentence from:

"We add behavior the scope by attaching methods to the $scope object."

to:

"We add behavior to the scope by attaching methods to the $scope object."

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[aevange]

Andre Evangelesta