Fixed typo in documentation.

docs/manual/src/docs/asciidoc/_includes/servlet/preface/java-configuration.adoc

@@ -679,7 +679,7 @@ We can override this quite simply to alter the way granted authorities are deriv
 
 ```java
 static class GrantedAuthoritiesExtractor extends JwtAuthenticationConverter {
-    protected Collection<GrantedAuthorities> extractAuthorities(Jwt jwt) {
+    protected Collection<GrantedAuthority> extractAuthorities(Jwt jwt) {
         Collection<String> authorities = (Collection<String>)
                 jwt.getClaims().get("mycustomclaim");