Add empty lines around callout text

Co-authored-by: eitsupi <[email protected]>

use/singularity.md

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 Rocker images can be imported and run using Singularity, with optional custom password support.
 
 :::{.callout-note}
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 In this guide, *Singularity* can refer to either [SingularityCE](https://sylabs.io/singularity/) or [Apptainer](https://apptainer.org/).
 While Apptainer is generally [compatible with Singularity commands and environment variables](https://apptainer.org/docs/user/latest/singularity_compatibility.html), Apptainer users may wish to replace the `singularity` command with `apptainer`, and `SINGULARITY` with `APPTAINER` in environment variables.
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 :::
 
 ## Importing a Rocker Image