docs(fix): fix spelling of consensus in CONTRIBUTING.md

[notmandatory]

Description

Small change to fix misspelling of "consensus" in #521.

Notes to the reviewers

I've installed harper-ls in my new editor (helix) to catch spelling errors sooner.

@noahjoeris caught this typo in bitcoindevkit/bdk#2328.

Before submitting

• I followed the contribution guidelines
• This PR breaks the existing API

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.66%. Comparing base (a704a07) to head (284fb1e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #576   +/-   ##
=======================================
  Coverage   81.66%   81.66%           
=======================================
  Files          25       25           
  Lines        6339     6339           
  Branches      302      302           
=======================================
  Hits         5177     5177           
  Misses       1055     1055           
  Partials      107      107           

Flag	Coverage Δ
rust	81.66% <ø> (ø)

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[noahjoeris]

ACK 284fb1e