Polish – Fix grammar mongo-encryption documentation

What & Why:

This PR polishes the mongo-encryption documentation by fixing a grammatical glitch ("a the MongoJsonSchema") and rephrasing the usage sentence for clarity and consistency with Spring Data documentation style.

Changes:

• Replaced grammar error "a the MongoJsonSchema"→ "a MongoJsonSchema."

• Re‐worded sentence to:

To use a MongoJsonSchema, you must supply it together with AutoEncryptionSettings which can be done eg. via a MongoClientSettingsBuilderCustomizer.

Impact:

• Docs‐only; no functional or API changes.

Verification:

• ./mvnw clean verify completes without errors (including spring-javaformat:validate).
• Generated Javadoc renders correctly.

Checklist:

• ./mvnw clean verify locally.

No tests or source files were affected other than the Javadoc.