correcting sticky footer example

[paulsmithkc]

The sticky footer example contained a few errors listed below:

• The example CSS did not include the required CSS styles for the HTML and BODY tags.
• The example CSS set the flex-shrink on the header and footer, but not flex-grow
• Improper indentation on the closing div tag in the HTML.
• The textarea for the example CSS was too short, requiring resizing to see the full example code.
• The preview area was too tall, making it hard to view the example code.
• The page did not properly separate styles used for demonstration, and styles used in the recipe.