[objc] fix documentation of API authorization in README files (#15347)

fix hierarchy level + align wording across generators

Author: tiffmaelite

modules/openapi-generator/src/main/resources/objc/README.mustache

@@ -118,9 +118,10 @@ Class | Method | HTTP request | Description
 
 ## Documentation For Authorization
 
-{{^authMethods}} All endpoints do not require authorization.
-{{/authMethods}}{{#authMethods}}{{#last}} Authentication schemes defined for the API:{{/last}}{{/authMethods}}
-{{#authMethods}}## {{{name}}}
+{{^authMethods}}Endpoints do not require authorization.{{/authMethods}}
+{{#hasAuthMethods}}Authentication schemes defined for the API:{{/hasAuthMethods}}
+{{#authMethods}}
+### {{{name}}}
 
 {{#isApiKey}}- **Type**: API key
 - **API key parameter name**: {{{keyParamName}}}

samples/client/petstore/objc/core-data/README.md

@@ -123,13 +123,14 @@ Class | Method | HTTP request | Description
 ## Documentation For Authorization
 
 
-## api_key
+Authentication schemes defined for the API:
+### api_key
 
 - **Type**: API key
 - **API key parameter name**: api_key
 - **Location**: HTTP header
 
-## petstore_auth
+### petstore_auth
 
 - **Type**: OAuth
 - **Flow**: implicit

samples/client/petstore/objc/default/README.md

@@ -123,13 +123,14 @@ Class | Method | HTTP request | Description
 ## Documentation For Authorization
 
 
-## api_key
+Authentication schemes defined for the API:
+### api_key
 
 - **Type**: API key
 - **API key parameter name**: api_key
 - **Location**: HTTP header
 
-## petstore_auth
+### petstore_auth
 
 - **Type**: OAuth
 - **Flow**: implicit