Remove the `markdown-link-check-disable` flag

[JoshuaBatty]

We added link check disable and enable flags around the following section in order to release the beta-4 PR. Once these links are live we should remove these flags.

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The `beta-4` channel is a published TOML file describing the toolchain that is compatible with our [beta-4 testnet](https://fuel-labs.ghost.io/announcing-beta-4-testnet/). This toolchain should be used to interact with and build on the testnet. The components to be installed can be found [here](https://github.com/FuelLabs/fuelup/blob/gh-pages/channel-fuel-beta-4.toml).
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