fix typo of 'mapping()'

Author: basemkhirat

readme.md

@@ -341,7 +341,7 @@ ES::index("my_index")->create();
 ```php
 ES::index("my_index")->create(function($index){
 
-    $index->shards(5)->replicas(1)->mappping([
+    $index->shards(5)->replicas(1)->mapping([
         'my_type' => [
             'properties' => [
                 'first_name' => [
@@ -360,7 +360,7 @@ ES::index("my_index")->create(function($index){
 
 ES::create("my_index", function($index){
 
-      $index->shards(5)->replicas(1)->mappping([
+      $index->shards(5)->replicas(1)->mapping([
           'my_type' => [
               'properties' => [
                   'first_name' => [