Bump OpenAPIKit - fixes a server variable parsing issue (#233)

Bump OpenAPIKit - fixes a server variable parsing issue

### Motivation

Fixes #228.

### Modifications

Bumps OpenAPIKit to a version with the fix.

### Result

OpenAPI docs that use server variables don't fail generation.

### Test Plan

Verified manually on an OpenAPI doc locally.


Reviewed by: gjcairo

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 
     ✖︎ pull request validation (integration test) - Build finished. 

#233

Author: czechboy0

Package.swift

@@ -64,7 +64,7 @@ let package = Package(
         // Read OpenAPI documents
         .package(
             url: "https://github.com/mattpolzin/OpenAPIKit.git",
-            exact: "3.0.0-beta.2"
+            exact: "3.0.0-beta.3"
         ),
         .package(
             url: "https://github.com/jpsim/Yams.git",