Add type overrides key to the docs (#788)

### Motivation

In #774 and #764 we missed adding the docs for the new key in the config
file.

### Modifications

Added the missing docs.

### Result

Type overrides property documented.

### Test Plan

N/A

Author: czechboy0

Sources/swift-openapi-generator/Documentation.docc/Articles/Configuring-the-generator.md

@@ -45,6 +45,8 @@ The configuration file has the following keys:
     - `defensive` (default): Produces non-conflicting Swift identifiers for any OpenAPI identifiers. Check out [SOAR-0001](https://swiftpackageindex.com/apple/swift-openapi-generator/documentation/swift-openapi-generator/soar-0001) for details.
     - `idiomatic`: Produces more idiomatic Swift identifiers for OpenAPI identifiers. Might produce name conflicts (in that case, switch back to `defensive`). Check out [SOAR-0013](https://swiftpackageindex.com/apple/swift-openapi-generator/documentation/swift-openapi-generator/soar-0013) for details.
 - `nameOverrides` (optional): a string to string dictionary. Allows customizing how individual OpenAPI identifiers get converted to Swift identifiers.
+- `typeOverrides` (optional): Allows replacing a generated type with a custom type.
+    - `schemas` (optional): a string to string dictionary. The key is the name of the schema, the last component of `#/components/schemas/Foo` (here, `Foo`). The value is the custom type name, such as `CustomFoo`. Check out details in [SOAR-0014](https://swiftpackageindex.com/apple/swift-openapi-generator/documentation/swift-openapi-generator/soar-0014).
 - `featureFlags` (optional): array of strings. Each string must be a valid feature flag to enable. For a list of currently supported feature flags, check out [FeatureFlags.swift](https://github.com/apple/swift-openapi-generator/blob/main/Sources/_OpenAPIGeneratorCore/FeatureFlags.swift).
 
 ### Example config files