Update methodology.rst

[joel-mcmurry]

Proposed changes

Changed language in documentation of S-learner to reflect the fact that mean conditional response $\mu(x,w)$ is defined under "treatment," not under "treatment effect." Treatment effect is estimated as difference $\hat{\mu}(x,1) - \hat{\mu}(x,0)$. See (Kunzel et al 2019, pp 3-4).

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

[t-tte]

Good catch