docs: remove reference to the deprecated uiSchema classNames proper…

…ty (rjsf-team#3294)

In the v5 migration guide, it is explained that `classNames` property within
uiSchema is now deprecated in favor of `ui:classNames` property. In developer
use the legacy classNames, a deprecation warning is displayed within the console
. But currently, in the uiSchema section within the quickstart guide,
documentation still refer to this deprecated property that is weird for
newcomers which will bootstrap a project with deprecation warnings.

Co-authored-by: Heath C <51679588+heath-freenome@users.noreply.github.com>

docs/api-reference/uiSchema.md

@@ -90,7 +90,7 @@ See [Custom Widgets and Fields](https://react-jsonschema-form.readthedocs.io/en/
 
 ### classNames
 
-The uiSchema object accepts a `classNames` property for each field of the schema:
+The uiSchema object accepts a `ui:classNames` property for each field of the schema:
 
 ```tsx
 import { UiSchema } from "@rjsf/utils";

docs/quickstart.md

@@ -63,7 +63,7 @@ const schema: RJSFSchema = {
 };
 
 const uiSchema: UiSchema = {
-  classNames: "custom-css-class"
+  "ui:classNames": "custom-css-class"
 };
 
 render((
@@ -94,10 +94,10 @@ const schema: RJSFSchema = {
 
 const uiSchema: UiSchema = {
   name: {
-    classNames: "custom-class-name"
+    "ui:classNames": "custom-class-name"
   },
   age: {
-    classNames: "custom-class-age"
+    "ui:classNames": "custom-class-age"
   }
 }