Docs should state default value for Resource Server validation clock skew is 60 seconds

[Thopap]

The documentation for the Oauth2 Resource Server validation (https://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#oauth2resourceserver-validation) contain a remark that the default configuration of 30 seconds clock is configured.

Two remarks on this:

1. The current implemented default seems to 60 seconds JwtTimestampValidator
   Either the documenation or the code need be changed.
2. When using the spring boot starter, a configuration property to change the default might make sense.

[jzheaux]

Good catch, @Thopap! I realize the chance is pretty quick; would you be interested in submitting a PR?