Fix loading example in opus dataset cards
This PR:
- fixes the examples to load the datasets, with the corrected dataset name, in their dataset cards for:
  - opus_dgt
  - opus_paracrawl
  - opus_wikipedia
- fixes their dataset cards with the missing required information: title, data instances/fields/splits
- enumerates the supported languages
- adds a missing citation reference for opus_wikipedia

Related to:
- #4806