Add hint to MSP conversions from TXT to metabolomics trainings

Sometimes MSP files have to be explicitly converted to `msp` after upload as they are text based and Galaxy by default takes them as `txt` files. We should add a hint about this to the training.

Locate the metabolomics training pages in the training material and read how uploaded MSP files are described. Add a hint that text-based MSP files may need to be explicitly converted to `msp` after upload because Galaxy can identify them as `txt`; the training should clearly communicate this conversion step.

Written by the indexing model from the issue text.