Minor README fixes

[blepping]

i fixed a few small typos/grammar issues in the README.

this part still doesn't sound quite right but i didn't want to change it too much:

(It is cumbersome to search for a compatible xformers with your current PyTorch. You can simply add xformers to your PyTorch installation using the command shown above.)

maybe something like this would work:

(If it is cumbersome to find a version of xformers compatible with your current PyTorch version, you can simply add xformers to your conda environment using the command shown above.)

or

(It may be cumbersome to find a version of xformers compatible with your current PyTorch version. If so, you can simply add xformers to your conda environment using the command shown above.)

not completely sure what the best way to phrase it would be because we are talking about installing it for the PyTorch within the conda environment not a normal Python environment or the global Python installation.