[mini] fix documentation on how to build the documentation

[SeverinDiederichs]

Due to a missing space, the code block on how to build the documentation did not appear.
This is fixed in this small PR.

Additionally, since all core functions are now OpenMP accelerated, I removed the deprecated statement that the code is not fully accelerated.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable