Update article.md

'"pro" and "contra" -> 'pros and cons'.

'pros and cons' is a much more typical way of saying it. But "pro" and "contra" is interesting. I hadn't previously known what "cons" stood for, though now it seems obvious.

Also, punctuation change.

2-ui/1-document/11-coordinates/article.md

@@ -250,4 +250,4 @@ Any point on the page has coordinates:
 
 Window coordinates are great to use with `position:fixed`, and document coordinates do well with `position:absolute`.
 
-Both coordinate systems have their "pro" and "contra", there are times we need one or the other one, just like CSS `position` `absolute` and `fixed`.
+Both coordinate systems have their pros and cons; there are times we need one or the other one, just like CSS `position` `absolute` and `fixed`.