Polish Authorization Server Sample

Change primary user's username to "subject" to align with
oauth2resourceserver sample.

samples/boot/oauth2authorizationserver/src/main/java/sample/JwkSetConfiguration.java

@@ -146,7 +146,7 @@ class UserConfig extends WebSecurityConfigurerAdapter {
 	public UserDetailsService userDetailsService() {
 		return new InMemoryUserDetailsManager(
 				User.withDefaultPasswordEncoder()
-					.username("user")
+					.username("subject")
 					.password("password")
 					.roles("USER")
 					.build());