docs: clarify first project run steps

Add a `cd my_project` step before running the program, briefly note
that Deno infers the `run` subcommand from a file path, and rephrase
the `main.ts` description so it isn't pinned to a specific Deno
version.

Author: bartlomieju

runtime/getting_started/first_project.md

@@ -35,22 +35,31 @@ my_project
 
 A `deno.json` file is created to
 [configure your project](/runtime/fundamentals/configuration/), and two
-TypeScript files are created; `main.ts` and `main_test.ts`. As of Deno 2.8 the
-`main.ts` file contains a small HTTP server built on
-[`Deno.serve`](/api/deno/~/Deno.serve) — it shows off Deno's built-in HTTP
-server, `Response.json()`, and TypeScript working out of the box. The handler is
-exported and guarded by `import.meta.main`, so `main_test.ts` can import and
-call it directly without binding to a port.
+TypeScript files are created; `main.ts` and `main_test.ts`. The `main.ts` file
+contains a small HTTP server built on [`Deno.serve`](/api/deno/~/Deno.serve) —
+it shows off Deno's built-in HTTP server, `Response.json()`, and TypeScript
+working out of the box. The handler is exported and guarded by
+`import.meta.main`, so `main_test.ts` can import and call it directly without
+binding to a port.
 
 ## Run your project
 
+Move into the new project directory:
+
+```bash
+cd my_project
+```
+
 You can run this program with the following command:
 
 ```bash
 $ deno main.ts
 Listening on http://localhost:8000/
 ```
 
+When you pass a file path directly, Deno infers the `run` subcommand, so
+`deno main.ts` is equivalent to `deno run main.ts`.
+
 Open the URL in your browser to see the response.
 
 ## Run your tests