Fix documentation links in examples

[1ace]

Objective

Fix documentation links in examples (2d_shapes & 3d_shapes).

Solution

The two links currently render as plain text (see links above); changing [label][target] to [label](target) seems like the right fix as this is the format used for links elsewhere, but I haven't figured out how to render them locally to verify.

Testing

Not tested; I haven't figured out how to render these locally, but at worst this will be a no-op change so let's see 😅

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[dloukadakis]

It seems the website is not handling rustdoc links properly. [][] is already valid rustdoc syntax for links.

We should probably consider rewriting these with compatibility with the website in mind or improve the website so it handles rustdoc links to some extent.