fix doc for application credentials (#381)

Author: k0ka

doc/services/identity/v3/application-credentials.rst

@@ -4,14 +4,14 @@ Application Credentials
 Add application credential
 --------------------------
 
-.. sample:: Identity/v3/application_credentials/add_application_credentials.php
+.. sample:: Identity/v3/application_credentials/add_application_credential.php
 
 Show application credential details
 -----------------------------------
 
-.. sample:: Identity/v3/application_credentials/show_application_credentials.php
+.. sample:: Identity/v3/application_credentials/show_application_credential.php
 
 Delete application credential
 -----------------------------
 
-.. sample:: Identity/v3/application_credentials/delete_application_credentials.php
+.. sample:: Identity/v3/application_credentials/delete_application_credential.php