Update README.md for consistency with solution.css (#537)

Since commit 65b831b changed `container`'s padding,
the instruction of padding in `README.md` should be changed as well.

Author: zhangsherry

flex/07-flex-layout-2/README.md

@@ -31,5 +31,5 @@ Note: The emojis may instead show up as one or several text symbols (e.g. &#9734
 - The sidebar is 300px wide (and it doesn't shrink).
 - The sidebar links are size 24px, are white, and do not have the underline text decoration.
 - The sidebar has 16px padding.
-- There is 32px padding around the 'cards' section.
+- There is 48px padding around the 'cards' section.
 - The cards are arranged horizontally, but wrap to multiple lines when they run out of room on the page.