DOC: sparse.linalg: Fix lobpcg docstring.

Change the markup of the "Acknowledgements" title from a section heading
to emphasis markers within the "Notes" section.

The docstring standard doesn't include an "Acknowledgements" section,
so including one in a docstring generates a warning when building the
docs, and the section is not included in the HTML output.
Merge pull request #3892 from WarrenWeckesser/doc-lobpcg

DOC: sparse.linalg: Fix lobpcg docstring.