Use Oxford comma properly in getex

[Andrew-Chen-Wang]

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Description of change

I believe the original implementation was trying to use an Oxford comma in the exception.

[chayim]

Thanks again @Andrew-Chen-Wang you're absolutely right. Much obliged as always! Great stuff, I love that someone's looking at our docs.

[Andrew-Chen-Wang]

@chayim definitely helps to have someone looking at it! Truthfully though, I'm simply working on updating aioredis to match with redis-py. Also I'm not sure if this is too much to ask for, but do you mind adding hacktoberfest-accepted to some of these PRs? If it's too much of a burden, totally understand (seems like it was never done in the past here).