Update 2016-05-31-rl.markdown

[brandonmburroughs]

Hey Andrej, I noticed what I think is a small typo. In the Trainable Memory I/O section, it says

For example, a Neural Turing Machine has a memory tape that they it read and write from.

I think it should be

For example, a Neural Turing Machine has a memory tape that they read and write from.

I submitted this PR to fix it, but feel free to just change it manually!

Great article by the way! I have little background in Reinforcement Learning and this really helped, especially the code sample. Thanks for the help!