OS101 various modules and lessons: minor consistency issue

[lauracion]

The substantive "data" is singular and plural throughout modules 1 to 3.

For example, there are a few "data is" singular in Module 1, Lesson 3, but two "data are" in Module 2, lesson 1. In Module 3, lesson 1 the use of the plural and the singular seems to be mixed.

This is minor, but distracting to me 😄