Fix spelling of "opinionated" in README

[mbomb007]

From https://github.com/curso-r/chess

{chess} is an opinionated wrapper for R around python-chess, an amazing library created by Niklas Fiekas. It allows users to read and write PGN files as well as create and explore game trees such as the ones seen in chess books.

The README for this project says:

Oppinionated wrapper to use python-chess from the R programming language

[cmerkle1]

Hello! I'm a new contributor and would love to start tackling some of these issues starting from the top. Going to go ahead and take care of this simple fix now.