docs: fix code block formatting in 'user-guide.rst'

[mattseymour]

Fix a formatting error on a code block which is formatting as a block of quoted text.

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[mattseymour]

@googlebot I signed it!

[tseaver]

We need to get a fix merged for #795 before we can merge this PR.

[tseaver]

@mattseymour Thanks very much for the patch!