Saddle-points: fix formatting of instructions

[JDeeth]

First line of demo grid is interpreted as a code block and the rest as paragraph text. Wrapping the whole thing in code tags forces it to be shown in fixed-width text instead.

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[senekor]

I think this document originally comes from problem specs. Let's fix it now though instead of waiting until I or someone else finds the time to sync properly.

[senekor]

@JDeeth Thanks!