Rename sampling statement to distribution statement

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] If a user-facing facing change was made, the documentation PR is here: https://github.com/stan-dev/docs/pull/777
  • [ ] OR, no user-facing changes were made

Release notes

Rename sampling statement (~) to distribution statement (~) to be consistent with the docs changes in https://github.com/stan-dev/docs/pull/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[bot]]

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

```diff @@ Coverage Diff @@ ## master #1426 +/- ## ======================================= Coverage 89.83% 89.83% ======================================= Files 63 63 Lines 10484 10484 ======================================= Hits 9418 9418 Misses 1066 1066 ``` | [Files](https://app.codecov.io/gh/stan-dev/stanc3/pull/1426?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev) | Coverage Δ | | |---|---|---| | [src/analysis\_and\_optimization/Pedantic\_analysis.ml](https://app.codecov.io/gh/stan-dev/stanc3/pull/1426?src=pr&el=tree&filepath=src%2Fanalysis_and_optimization%2FPedantic_analysis.ml&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev#diff-c3JjL2FuYWx5c2lzX2FuZF9vcHRpbWl6YXRpb24vUGVkYW50aWNfYW5hbHlzaXMubWw=) | `93.30% <ø> (ø)` | | | [src/frontend/Typechecker.ml](https://app.codecov.io/gh/stan-dev/stanc3/pull/1426?src=pr&el=tree&filepath=src%2Ffrontend%2FTypechecker.ml&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev#diff-c3JjL2Zyb250ZW5kL1R5cGVjaGVja2VyLm1s) | `91.89% <100.00%> (ø)` | | | [src/frontend/Semantic\_error.ml](https://app.codecov.io/gh/stan-dev/stanc3/pull/1426?src=pr&el=tree&filepath=src%2Ffrontend%2FSemantic_error.ml&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev#diff-c3JjL2Zyb250ZW5kL1NlbWFudGljX2Vycm9yLm1s) | `90.99% <83.33%> (ø)` | |