minor corrections to the JS Lesson 3 tutorial

Hi all, have made the following small corrections:

* corrected the mentioned '.complete' class to '.completed' to match the actual stylesheet
* change the word 'value' to 'text' to prevent confusion (may otherwise lead to students using 'val()')
* added forward slash missing from a closing div tag

Other suggested changes that I have not made here are:

* change the variable name random_position to randomPosition to keep inline with Javascript convention
* add 'cursor: pointer' to the '.label' selector in the stylesheet, for better user experience when testing