Add import numpy as np in concept.md

[srijanreddy98]

In line 87 of concepts.md np is used but not imported. This just adds that import statement.

action = np.random.choice(action_list, p=prob_list)

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[srijanreddy98]

@googlebot I signed it!

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[lanctot]

Good catch!

[lanctot]

We have integrated this PR internally, so this will go out on the next push to github. Thanks!

[srijanreddy98]

Happy to help.

[lanctot]

Oh there was need to close it. Our push to github does that and associates the commits with it, so I like to keep it up until we do the push. (That way you and everyone else get notified when it's actually merged.)