Fix Security OAuth2 Client documentation

This commit fixes incorrect section level and sample configuration properties.

See gh-10615

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -2836,7 +2836,7 @@ explicitly configure the paths that you do want to override.
 [[boot-features-security-oauth2]]
 === OAuth2
 
-=== Client
+==== Client
 If you have `spring-security-oauth2-client` on your classpath you can take advantage of
 some auto-configuration to make it easy to set up an OAuth2 Client. This configuration
 makes use of the properties under `OAuth2ClientProperties`.
@@ -2858,7 +2858,7 @@ You can register multiple OAuth2 clients and providers under the
 							provider: my-oauth-provider
 							scope: user
 							redirect-uri: http://my-redirect-uri.com
-							authentication-method: basic
+							client-authentication-method: basic
 							authorization-grant-type: authorization_code
 						my-client2:
 							client-id: abcd
@@ -2867,7 +2867,7 @@ You can register multiple OAuth2 clients and providers under the
 							provider: my-oauth-provider
 							scope: email
 							redirect-uri: http://my-redirect-uri.com
-							authentication-method: basic
+							client-authentication-method: basic
 							authorization-grant-type: authorization_code
 				provider:
 					my-oauth-provider: