[docs] fix a grammar issue in the sdk readme

Change-Id: I566740cc36cbc85b16ece550b31481755b86524a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/298720
Reviewed-by: Michael Thomsen <[email protected]>
Commit-Queue: Michael Thomsen <[email protected]>
Auto-Submit: Devon Carew <[email protected]>

Author: devoncarew

sdk/api_readme.md

@@ -2,15 +2,15 @@
 
 Welcome to the Dart API reference documentation, covering the
 [Dart core libraries](https://dart.dev/guides/libraries). These include:
-   
+
   * [dart:core](dart-core/dart-core-library.html): Core functionality such as
     strings, numbers, collections, errors, dates, and URIs.
   * [dart:io](dart-io/dart-io-library.html): I/O for non-web apps.
   * [dart:html](dart-html/dart-html-library.html): DOM manipulation for web apps
     (available only to web apps).
-  
-For most of the core libraries - except for `dart:core` - need to imported
-before they're available for use. For example:
+
+The core libraries - except for `dart:core` - must be imported before they're
+available for use:
 
 ```dart
 import 'dart:math';
@@ -20,8 +20,8 @@ Additionally, you can find Dart packages at [pub.dev](https://pub.dev).
 
 ## Language docs
 
-The main site for learning and using Dart is
-[dart.dev](https://dart.dev). Check out these pages:
+The main site for learning and using Dart is [dart.dev](https://dart.dev).
+Check out these pages:
 
   * [Dart overview](https://dart.dev/overview)
   * [Dart language documentation](https://dart.dev/language)