Fix order of 2 sections in the Validation chapter of the reference manual

[ZeroCyan]

There is currently a wrong order of webpages in the documentation under https://docs.spring.io/spring-framework/reference/core/validation.html

The page "Resolving Codes to Error Messages" mentions the page "Data Binding" (see first sentence: "We covered databinding and validation."), but is presented before the latter.

Current order:

• Validation by Using Spring’s Validator Interface
• Resolving Codes to Error Messages
• Data Binding
• Spring Type Conversion

Expected order:

• Validation by Using Spring’s Validator Interface
• Data Binding
• Resolving Codes to Error Messages
• Spring Type Conversion

[sbrannen]

This has been merged into 6.1.x and main.

Thanks