Fix example (#33)

Author: thesebas

matchers.md

@@ -265,8 +265,8 @@ it("expects Foo to receive ::message() with the correct param only once", functi
 ```php
 it("expects Foo to receive ::message() with the correct param a specified number of times", function() {
     expect(Foo::class)->toReceive('::message')->with('My Message')->times(2);
-    $foo::message('My Message');
-    $foo::message('My Message');
+    Foo::message('My Message');
+    Foo::message('My Message');
 });
 ```