bug #1056 fix(Apps): use /orgs/ORG/installation (ellisio)

This PR was merged into the 3.4.x-dev branch.

Discussion
----------

According to [the documentation](https://docs.github.com/en/rest/reference/apps#get-an-organization-installation-for-the-authenticated-app) this should be `/orgs/ORG/installation`, not `/org/ORG/installation`.

Using the singular version results in `Bad credentials` when using a valid JWT for the GitHub App.

Commits
-------

4d53365 fix(Apps): use /orgs/ORG/installation

lib/Github/Api/Apps.php

@@ -82,7 +82,7 @@ public function getInstallationForOrganization($org)
     {
         $this->configurePreviewHeader();
 
-        return $this->get('/org/'.rawurldecode($org).'/installation');
+        return $this->get('/orgs/'.rawurldecode($org).'/installation');
     }
 
     /**

test/Github/Tests/Api/AppTest.php

@@ -66,7 +66,7 @@ public function shouldGetInstallationForOrganization()
         $api = $this->getApiMock();
         $api->expects($this->once())
             ->method('get')
-            ->with('/org/1234/installation')
+            ->with('/orgs/1234/installation')
             ->willReturn($result);
 
         $this->assertEquals($result, $api->getInstallationForOrganization('1234'));