Normalize fields for SE-0255

Update fields of SE-0255 to conform to proposal template directions

- Change 'Previous Proposal' to 'Previous Revision'
- Use canonical format for previous revisions
- Change 'Decision Notes' to 'Review'
- Add pitch and review links to review field
- Correct acceptance link to use the acceptance thread

The linked proposal is a previous revision of this proposal, not a
separate previous proposal. This change uses the correct field name.

SE proposal metadata extraction will begin extracting previous proposals
and review links. This PR allows correct metadata to be extracted.

Author: dempseyatgithub

proposals/0255-omit-return.md

@@ -5,8 +5,8 @@
 * Review Manager: [Ben Cohen](https://github.com/airspeedswift)
 * Status: **Implemented (Swift 5.1)**
 * Implementation: [apple/swift#23251](https://github.com/apple/swift/pull/23251)
-* Decision Notes: [Rationale](https://forums.swift.org/t/se-0255-implicit-returns-from-single-expression-functions/22544/113)
-* Previous Proposal: [SE-NNNN](https://github.com/DevAndArtist/swift-evolution/blob/single_expression_optional_return/proposals/nnnn-single-expression-optional-return.md)
+* Previous Revision: [1](https://github.com/DevAndArtist/swift-evolution/blob/single_expression_optional_return/proposals/nnnn-single-expression-optional-return.md)
+* Review: ([pitch](https://forums.swift.org/t/pitch-implicit-returns-from-single-expression-functions/21898)), ([review](https://forums.swift.org/t/se-0255-implicit-returns-from-single-expression-functions/22544)), ([acceptance](https://forums.swift.org/t/accepted-se-0255-implicit-returns-from-single-expression-functions/23581/1))
 
 ## Introduction