[README] Update the SSWG badge (#714)

### Motivation

This project has moved into the Incubating stage of the SSWG incubation
process:
https://www.swift.org/sswg/incubated-packages.html#incubating-projects

### Modifications

Updated the README badge to reflect that.

### Result

Accurate badge.

### Test Plan

Previewed locally.

Author: czechboy0

README.md

@@ -1,6 +1,6 @@
 # Swift OpenAPI Generator
 
-[![](https://img.shields.io/badge/sswg-sandbox-lightgrey.svg)](https://www.swift.org/sswg/)
+[![](https://img.shields.io/badge/sswg-incubating-yellow.svg)](https://www.swift.org/sswg/)
 [![](https://img.shields.io/badge/docc-read_documentation-blue)](https://swiftpackageindex.com/apple/swift-openapi-generator/documentation)
 [![](https://img.shields.io/github/v/release/apple/swift-openapi-generator)](https://github.com/apple/swift-openapi-generator/releases)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-openapi-generator%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/apple/swift-openapi-generator)