Update libcudf documentation build command in DOCUMENTATION.md

[davidwendt]

Description

Updates the instruction to build the libcudf documentation files in DOCUMENTATION.md. The cmake --build . --target docs_cudf will invoke the appropriate make tool as setup when cmake was configured for building libcudf.

Closes #11719

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.