minor fixes vue workshop ch4 ch5

[egentilucci]

Thank you so much for the workshop, it was really helpful to start with vue!

I found some minor bumps and I would like to propose some changes:

ch4 changes:

• adjust "slot" attribute usage
• found a typo on the word "thhis"

ch5 changes:

• changed the variable used for the validation of the "Submit" button, using "nameRules" didn't work for me, I used "valid" instead
• removed ";" on some code snippets because it was creating error when copying