fix(install): fix incorrect concurrency option when creating a dev install (#15)

Creating a development install would result in an error because the dashes of the concurrency option that is passed through npm to lerna was missing the two dashes.
This bug was introduced in a refactoring in a593c42.

Author: hlxid

src/install/development.ts

@@ -185,6 +185,6 @@ async function installNPMDependencies(nodecgIODir: string) {
 
 async function buildTypeScript(nodecgIODir: string, concurrency: number) {
     logger.info("Compiling nodecg-io...");
-    await runNpmBuild(nodecgIODir, "--", "concurrency", concurrency.toString());
+    await runNpmBuild(nodecgIODir, "--", "--concurrency", concurrency.toString());
     logger.success("Compiled nodecg-io.");
 }