Update resource-server.adoc

Fix typo in the code sample

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/resource-server.adoc

@@ -297,7 +297,7 @@ We can override this quite simply to alter the way granted authorities are deriv
 [source,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");