Merge bitcoindevkit/bdk#981: Fixed typos in CONTRIBUTING.md

c7a045fa54a8183899d90730249c06c3f538b027 Fixed typos in CONTRIBUTING.md (Jon Marrs)

Pull request description:

  ### Description

  Fixed typos in CONTRIBUTING.md

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  thunderbiscuit:
    ACK c7a045fa54a8183899d90730249c06c3f538b027.

Tree-SHA512: de66f4be7c7fd608f5a74e51c5666f4fe6b962c5a8345c08c7b1da2551f81b9adfb28c58f0991ac6c985fa5c31f3eb7d17f99edf136b04945cefa24b28eaee19