Rename sampling statement to distribution statement

[WardBrian]

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here: sampling statement -> distribution statament docs#777
  • OR, no user-facing changes were made

Release notes

Rename sampling statement (~) to distribution statement (~) to be consistent with the docs changes in stan-dev/docs#777

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.83%. Comparing base (2c42646) to head (9f9f557).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1426   +/-   ##
=======================================
  Coverage   89.83%   89.83%           
=======================================
  Files          63       63           
  Lines       10484    10484           
=======================================
  Hits         9418     9418           
  Misses       1066     1066           

Files	Coverage Δ
src/analysis_and_optimization/Pedantic_analysis.ml	93.30% <ø> (ø)
src/frontend/Typechecker.ml	91.89% <100.00%> (ø)
src/frontend/Semantic_error.ml	90.99% <83.33%> (ø)

[avehtari]

Looks good (and better than my PR)