Updated image logo URL.

README.rst

@@ -2,7 +2,7 @@
     :target: http://docs.gprmax.com/en/latest/?badge=latest
     :alt: Documentation Status
 |
-.. image:: /docs/source/images/gprMax_logo_small.png
+.. image:: https://github.com/gprMax/gprMax/blob/master/docs/source/images/gprMax_logo_small.png
     :target: http://www.gprmax.com
 
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