Fix grammatical errors in contribution guide (#95454)
Fixed following errors in contribution guide.
"deep neural networks using a **on** tape-based autograd systems." to "deep neural networks **using a tape-based** autograd systems."
"the best entrance **point** and are great places to start." to "the best entrance **points** and are great places to start."
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95454
Approved by: https://github.com/ezyang
