[css-align] Editorial: the 'stretch' definition moved, but the section titles weren't updated accordingly

NOTE: This mail is targeting the current editor draft of CSS Box Alignment, 
not its current working draft, which is fine regarding this.


In more details, my comments are:
- Section 3.1's title still mentions ‘strecth’ (yet the section doesn't 
define it)
- Section 3.3's title doesn’t mention ‘stretch’ (yet the section does define 
it)

I believe the section titles should be updated to match their content (or 
vice-versa).

Best regards,
François