chore: simple code example in README

README.md

 # mquery (placeholder name)
 A Rust library for handling PROMQL (and possibly MetricsQL) queries.
 
-# Usage Example
+## Usage Example
 ```rust
 use mquery::{Auth, QueryManager};
-use tokio::runtime::Runtime;
 
-dotenv::dotenv().expect("No .env file found in working dir");
-let url = std::env::var("VM_URL").expect("VM URL not found in env");
-let token = std::env::var("VM_TOKEN").expect("VM URL not found in env");
-let rt = Runtime::new().unwrap();
-let query = "total_http_requests";
-let _response = rt
-    .block_on(
-        QueryManager::new(url.parse().unwrap())
-            .auth(Auth::Bearer(token))
-            .query(query),
-    )
-    .expect("operation failed");
+#[tokio::main]
+async fn main() {
+    dotenv::dotenv().expect("No .env file found in working dir");
+    let url = std::env::var("VM_URL").expect("VM URL not found in env");
+    let token = std::env::var("VM_TOKEN").expect("VM URL not found in env");
+    let query = "total_http_requests";
+
+    let _response = QueryManager::new(url.parse().unwrap())
+                      .auth(Auth::Bearer(token))
+                      .query(query)
+                      .await
+                      .expect("operation failed");
 }
 ```