When I was in college we did this to a machine in the computer lab and changed analysis to ana....well, perhaps it's best left to your imagination. A few days later, one of my friends overheard someone talking about how horrified they were when a paper came back from their professor with the substituted word circled in red. Repeatedly.
Anyway, what you need to do is open a doc in Google Docs, then click on TOOLS>PREFERENCES. You should see a list with two columns or words and a check box beside each pair. These are auto-substitutions. If you scroll down, you should see where "your" and "you're" are entered. You can either click the "X" beside them on the right to delete the entry, or uncheck the box on the left to simply stop the auto-substitution.