[Doctrine] Fix markdown in multiple_entity_managers.rst

[aivus]

Fix broken markdown at the end of the multiple entity managers page

[javiereguiluz]

@aivus thanks for this contribution ... however, Oskar was right and this should be part of the .. caution. The real problem was that this line was using a TAB instead of white spaces. I spotted this while editing this in a text editor:

So, we've restored the indentation, but with white spaces, while merging. Cheers!