feat(proxy): update proxy config argument

Changes the flag for the proxy config in the `serve:json-server` script so that it no longer uses
the deprecated version of the flag.

Implements #9

src/ng-add/setup.ts

@@ -98,7 +98,7 @@ function addPackageJsonScript(options: Schema): Rule {
     );
     const serveScript = `npm run json-server & ng serve ${
       options.project
-    } --proxyConfig ${proxyPath}`;
+    } --proxy-config ${proxyPath}`;
     try {
       const packageJsonFile = tree.read(`package.json`);
       if (packageJsonFile) {