Update README.md

Author: electricessence

README.md

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 # Open.Disposable.ObjectPools
 A set of variations on ObjectPool implementations with differing underlying collections.
 
-[![NuGet](http://img.shields.io/nuget/v/Open.Disposable.ObjectPools.svg)](https://www.nuget.org/packages/Open.Disposable.ObjectPools/)
+[![NuGet](https://img.shields.io/nuget/v/Open.Disposable.ObjectPools.svg)](https://www.nuget.org/packages/Open.Disposable.ObjectPools/)