Fix broken SystemTextJson URL (#250)

README.md

@@ -70,7 +70,7 @@ The components within this repository are delivered as multi-targeted Nuget pack
   - uses Json.net to serialize the event bodies.
   - `FsCodec.Box.Codec`: Testing substitute for `FsCodec.NewtonsoftJson.Codec`, included in same package.
   - ([depends](https://www.fuget.org/packages/FsCodec.NewtonsoftJson) on `FsCodec`, `Newtonsoft.Json >= 11.0.2`, `TypeShape`, see [FsCodec repo](https://github.com/jet/FsCodec) for details)
-- `FsCodec.SystemTextJson` [![SystemTextJson Codec NuGet](https://img.shields.io/nuget/v/FsCodec.SystemTextJson.svg)]: Drop in replacement that allows one to target the .NET `System.Text.Json` serializer solely by changing the referenced namespace.
+- `FsCodec.SystemTextJson` [![SystemTextJson Codec NuGet](https://img.shields.io/nuget/v/FsCodec.SystemTextJson.svg)](https://www.nuget.org/packages/FsCodec.SystemTextJson/): Drop in replacement that allows one to target the .NET `System.Text.Json` serializer solely by changing the referenced namespace.
 
 ### Store libraries