Fix Debian Sources file name

Mathieu Parentrequested to merge
sathieu/gitlab:debian_source_s into master

What does this MR do and why?

Describe in detail what your merge request does and why.

While implementing Debian Sources endpoint, I noticed a typo: a missing s.

The debian docs specify sources, so we are updating to maintain consistency.

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- Source
+ Sources
Edited by Steve Abrams

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