Apply typos fixes from ISC sanity check

[sadielbartholomew]

@danny-lloyd helpfully conducted a sanity check of the new course Notebooks prior to their use later this week for the ISC course in: ncasuk/ncas-isc#208, during which he noticed a pair of repeated typos. These fixes should be made also here to the corresponding Notebooks.

[sadielbartholomew]

Will wait to do this until the lessons have been taught for this year's ISC, later this week, in case further issues crop up (hopefully not, but something minor at least is likely to be spotted).