fix minor typos

[miljan-aleksic]

Beside the fixed typos I would like to notice that there are inconsistencies with the rest of the docs.

• The code examples are not ES6
• Vue and Vuex mentions are written in lower case (fixed in PR)
• Some code examples are still in TODO state
• In general the speaking voice differs. Its obvious has been written by a different person
• There are no source files of the tutorial images

Is not my intention to criticize the efforts put into this tutorial but to point out those issues and offer my further assistance. I'm not native English speaker but could try improving the mentioned, if accepted.

[blake-newman]

@yyx990803 @vuejs/collaborators

Fixes look good to me, ill let @yyx990803 decide on the other points.