minimize README code example

[sasezaki]

As I said at #347 , and changed like 37d8420

It seems property $prophecy taken as mixed in phpstan. With level 10 would be error.

as example, see below analyse result

diff --git a/test/StaticAnalysis/Test/ObjectProphecy/ProphesizeTest.php b/test/StaticAnalysis/Test/ObjectProphecy/ProphesizeTest.php
index 76d7552..cce56f9 100644
--- a/test/StaticAnalysis/Test/ObjectProphecy/ProphesizeTest.php
+++ b/test/StaticAnalysis/Test/ObjectProphecy/ProphesizeTest.php
@@ -28,10 +28,7 @@ final class ProphesizeTest extends Framework\TestCase
 {
     use ProphecyTrait;

-    /**
-     * @var ObjectProphecy<Src\BaseModel>
-     */
-    private \Prophecy\Prophecy\ObjectProphecy $prophecy;
+    private $prophecy;

     protected function setUp(): void
     {
@@ -40,6 +37,7 @@ final class ProphesizeTest extends Framework\TestCase

     public function testCreateProphecyInSetUp(): void
     {
+        \PHPStan\dumpType($this->prophecy);
         $this->prophecy
             ->getFoo()
             ->willReturn('bar');

vendor/bin/phpstan analyse --configuration=phpstan-with-extension.neon --level=10 test/StaticAnalysis/Test/ObjectProphecy/ProphesizeTest.php
 1/1 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ ----------------------------------------------------------------------------------------------------------------------
  Line   ProphesizeTest.php
 ------ ----------------------------------------------------------------------------------------------------------------------
  31     Property JanGregor\Prophecy\Test\StaticAnalysis\Test\ObjectProphecy\ProphesizeTest::$prophecy has no type specified.
         🪪  missingType.property
  40     Dumped type: mixed
  41     Cannot call method getFoo() on mixed.
         🪪  method.nonObject

I removed it, because it is not mostly required as code example.

Follows #348
Related to #346
Fixes #348