fix: remove useless escape character in content-collections.mdx

[ArmandPhilippot]

Description (required)

Same as #10209 but with a different pattern:
I haven't noticed this pattern in the previous PR because I was looking for an exact matching... but I checked "**\/*.md" and "**/*.md" are both valid.
So, to avoid confusion whether the editor automatically removes the character or ESlint displays a warning, I removed the useless escape character in some loader patterns in content-collections.mdx.

Related issues & labels (optional)

• Suggested label: code snippet update

[astrobot-houston]

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[sarah11918]

Perfect, thank you!

[yanthomasdev]

Nice catch, thanks!