Fix documentation warning when rendering diff docstring

[ndgrigorian]

This PR fixes a warning caused by the docstring for diff incorrectly indenting the description of the output shape.

Closes #1849

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1855/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.19.0dev0=py310hdf72452_109 ran successfully.
Passed: 895
Failed: 0
Skipped: 119

[github-actions]

Array API standard conformance tests for dpctl=0.19.0dev0=py310hdf72452_110 ran successfully.
Passed: 895
Failed: 0
Skipped: 119

[coveralls]

coverage: 87.907%. remained the same
when pulling c8de265 on fix-diff-docstring
into f0f7837 on master.

[oleksandr-pavlyk]

The reported sphinx errors are gone indeed. One outstanding issue is

/home/runner/work/dpctl/dpctl/dpctl/tensor/_indexing_functions.py:docstring of dpctl.tensor._indexing_functions.take_along_axis:24: WARNING: undefined label: 'broadcasting'