GridCell proptypes has a typo

[valadas]

Describe the bug

console warns that the component proptypes are not defined properly

To Reproduce

Steps to reproduce the behavior:

1. Use a component that uses GridCell
2. Notice in console the error below

Expected behavior

Should not give error

Error log

Warning: Component GridCell declared PropTypes instead of propTypes. Did you misspell the property assignment?

Additional context

This is caused by a type in GridCell.PropTypes, it should be GridCell.propTypes (lowercase p) I am submitting a pull request in a minute