Fix variable in _prometheus.html.haml so it shows as a code block

What does this MR do and why?

When loading /admin/application_settings/metrics_and_profiling without a valid prometheus_multiproc_dir, a warning is supposed to show that your ENV variable or directory is not valid. This warning had an error where it would point to a variable that didn't exist, while it should've been pointing to a string, this MR fixes that.

Screenshots or screen recordings

How to set up and validate locally

Two things to validate:

(The first fails on the current master branch)

1. Remove the directory that environment variable prometheus_multiproc_dir points to (default: gitlab/tmp/prometheus_multiproc_dir/puma
2. Go to <gdk-url>/admin/application_settings/metrics_and_profiling
3. Expand Metrics - Prometheus, a warning should now show up (see screenshot above).

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1. (re)create the directory
2. Go to <gdk-url>/admin/application_settings/metrics_and_profiling
3. Expand Metrics - Prometheus, a warning should not show up.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Fixes #352055