[Bug Report] Typo in assertions

[drmeerkat]

Describe the bug
There is a typo in the assertion information in line 68 of minigrid/core/grid.py. "Column index {j}" should be "Column index {i}".

Code example

assert (
        0 <= i < self.width
), f"column index {j} outside of grid of width {self.width}"

should be

assert (
        0 <= i < self.width
), f"column index {i} outside of grid of width {self.width}"

Checklist

• I have checked that there is no similar issue in the repo (required)

[pseudo-rnd-thoughts]

Thanks could you make a PR for this?

[drmeerkat]

done!