[CrossFile] Fix class name in readme (#383)

It says you have to use a class CrossFile which doesn't even exist in the package. It must be XFile.
diff --git a/packages/cross_file/CHANGELOG.md b/packages/cross_file/CHANGELOG.md
index 508db91..a6ace9a 100644
--- a/packages/cross_file/CHANGELOG.md
+++ b/packages/cross_file/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.3.1+3
+
+* Fix example in README
+
 ## 0.3.1+2
 
 * Fix package import in README
diff --git a/packages/cross_file/README.md b/packages/cross_file/README.md
index f9bbbf5..15f3da9 100644
--- a/packages/cross_file/README.md
+++ b/packages/cross_file/README.md
@@ -13,7 +13,7 @@
 ```dart
 import 'package:cross_file/cross_file.dart';
 
-final file = CrossFile('assets/hello.txt');
+final file = XFile('assets/hello.txt');
 
 print('File information:');
 print('- Path: ${file.path}');
diff --git a/packages/cross_file/pubspec.yaml b/packages/cross_file/pubspec.yaml
index 79175d1..b8b62ca 100644
--- a/packages/cross_file/pubspec.yaml
+++ b/packages/cross_file/pubspec.yaml
@@ -2,7 +2,7 @@
 description: An abstraction to allow working with files across multiple platforms.
 repository: https://github.com/flutter/packages/tree/master/packages/cross_file
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+cross_file%22
-version: 0.3.1+2
+version: 0.3.1+3
 
 environment:
   sdk: ">=2.12.0 <3.0.0"