Fix typo.
Remove tab before sample codes.
Add description to hello-world.
Fix issues that have been pointed out.
Change attribute.

"use" was obsolete.
can use "instance" for component.
Change element name to lower-case.
Review

- indent
- not "==" but "==="
- not use "when" attribute
- "linstyle" property to "linejoin" and "linecap"
- when <set>...</set>, no return sentence.
Remove unnecessary space codes.
Reflect issues pointed out int the review.
Remove code tags.
Add property width and height for canvas.