Updated spanish files to be translated.
Translate 03a-The_Aggregation_Pipeline-Lecture. Closes #386
Translate 02a-Simple_Aggregation_Example-Lecture. Closes #385
Fixes according to comments from Jorge Puente-Sarrín. #385
Merge pull request #410 from juancarlosfarah/master

Translate 03a-The_Aggregation_Pipeline-Lecture. Closes #386