update site image for open graph and twitter cards

_config.yml

@@ -2,7 +2,7 @@ title: CircuitPython
 email: justin@adafruit.com
 description: >-
   The easiest way to program microcontrollers
-headline_image: "https://circuitpython.org/assets/images/CircuitPython_Hero.jpg"
+site_image: "https://circuitpython.org/assets/images/CircuitPythonLogo_Black.jpg"
 timezone: America/New_York
 twitter_username: circuitpython
 github_username:  adafruit

_includes/metadata.html

@@ -29,6 +29,6 @@
 <meta property="og:image" content="{{ "/assets/images/boards/large/" | append: page.board_image | absolute_url }}" />
 {% else %}
 <meta name="twitter:card" content="summary" />
-<meta property="og:image" content="{{ site.headline_image }}" />
+<meta property="og:image" content="{{ site.site_image }}" />
 {% endif %}
 <meta property="og:type" content="website" />