Fix typo in reference cache example code

[seate]

Fixed minor typo on example code
RedisCacheManager.build(RedisCacheWriter.class) to RedisCacheManager.builder(RedisCacheWriter.class)
Related Docs

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).

[pivotal-cla]

@seate Please sign the Contributor License Agreement!

Click here to manually synchronize the status of this Pull Request.

See the FAQ for frequently asked questions.

[pivotal-cla]

@seate Thank you for signing the Contributor License Agreement!

[mp911de]

Thank you for your contribution. That's merged and backported now.