Fix: add missing spectrometer PDF reference

[amisha1072006]

Fixes #11693

This PR adds the missing reference to the spectrometer PDF guide
where it was previously broken or absent.

Changes

• Added the correct spectrometer PDF link
• Ensured the reference points to a valid and accessible PDF

Testing

• Verified the link loads correctly in the browser

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[amisha1072006]

Hi!,
This PR only adds a missing spectrometer PDF reference and does not affect application logic.

Some CI checks appear to be failing, which seem unrelated to this change.
Please let me know if you’d like me to update anything. Thanks!

[shreyapawar17]

Hi, I would like to work on this issue. Can you please assign it to me?