doc: update docs/dart.md (#814)

### Current behavior
The current asynchronous login sample code has the following issues:
1. The `main` function tries to use `await userName()`, but `userName` is a string variable, not a function.
2. the `login` function should be called and `await` should be used to wait for its result.
### Expected behavior
The `main` function should correctly call the `login` function and use `await` to wait for the result, then print out the username.
### Sample code
Here is the corrected code:
```dart
Future<String> login() {
String userName = "Temidjoy";
return Future.delayed(
Duration(seconds: 4), () => userName
).
}
main() async {
print('Authenticating please wait...') The
String result = await login();
String result = await login(); print(result);
} f0da2a3397

docs/dart.html

@@ -699,7 +699,8 @@
 </span><span class="code-line"><span class="token comment">// 异步</span>
 </span><span class="code-line"><span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">async</span> <span class="token punctuation">{</span>
 </span><span class="code-line">  <span class="token function">print</span><span class="token punctuation">(</span><span class="token string-literal"><span class="token string">'Authenticating please wait...'</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
-</span><span class="code-line">  <span class="token function">print</span><span class="token punctuation">(</span><span class="token keyword">await</span> <span class="token function">userName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
+</span><span class="code-line">  <span class="token class-name">String</span> result <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">login</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
+</span><span class="code-line">  <span class="token function">print</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
 </span><span class="code-line"><span class="token punctuation">}</span>
 </span></code></pre>
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