Update README.md

Author: jdeigendesch

README.md

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 # ASP.NET Core Example
-<h4>This is an example of a REST API with ASP.NET Core v1.0, which is made via following tutorials:</h4>
+<h3>This is an example of a REST API with ASP.NET Core v1.0, which is made via following tutorials:</h3>
 
 Channel9: Creating REST API with ASP.NET Core (german): https://www.microsoft.com/germany/techwiese/know-how/video.aspx?id=msdn_de_67138