Fix broken links in reference documentation

[christophstrobl]

404 NOT_FOUND: https://docs.spring.io/spring-framework/docs/6.0.0/spring-framework-reference/core.html#expressions
404 NOT_FOUND: https://docs.spring.io/spring/docs/6.0.0/spring-framework-reference/core.html#expressions
404 NOT_FOUND: https://docs.spring.io/spring-framework/docs/6.0.0/spring-framework-reference/core.html#expressions-spel-compilation