Reflect relocation of osx-docbook-register in the community readme.

The build process documented in the README.md in the community folder pointed to a file called neo4j/manual/src/build/osx-register-docbook.
However, this file seems to have been moved to neo4j/manual/osx-docbook-register.

Updated the README.md in the community folder to reflect this change and point to the correct file.