Update marking-a-model-as-complete-incomplete.md

A typo.
Update adding-a-route-and-template.md

Add missing files to be included in index.html
Update displaying-the-number-of-incomplete-todos.md

A typo
Update accepting-edits.md

Missing extension
Update display-a-button-to-remove-completed-todos.md

Typos
Update toggle-all-todos.md

Typo
Merge pull request #507 from sqbell/patch-1

Correct typos and minor issues in Getting Started guide