docs(vignette): Add additional information on the importance of a FASTA file

[tonywu1999]

Motivation and Context

One user had a lot of trouble realizing how the FASTA file worked, even with the vignette. Upon first inspection, our vignettes do not mention anything about the importance of a FASTA file.

Changes

• Added a paragraph describing how to get the FASTA file from Uniprot and why it's important in the MSstatsPTM vignette

Testing

• Vignette builds fine.

Checklist Before Requesting a Review

• [x] I have read the MSstats contributing guidelines
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules