Update tasks.md

[nguyenelainana]

Corrected all grammatical and formatting errors in tasks.md

Example: Original: Open app.js and add an event lister to the window. Listen for the DOMContentLoaded event. The event handler should be an anonymous function.

Correction: Open app.js and add an event listener to the window. Listen for the DOMContentLoaded event. The event handler should be an anonymous function.

Link to commit: 06f6553

[nguyenelainana]

@kiffen-schutz and @snychka