Fix call to render to include mandatory argument

[orangethunder]

First of all, I don't really know Python, so apologies if there's a better way of doing this.

Inspecting the render method in the call being modified by this PR yields:

def render(mode):
    return env._render(mode, close=False)

I'm not sure where it's defined, but it has the mode argument as mandatory instead of optional, so I made this change. I can only assume that the gym package was on an older version when this was released initially?