Improve NuGet badge

- Fix casing.
- Add logo.

Author: martincostello

README.md

@@ -10,7 +10,7 @@
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 [![Build](https://github.com/grafana/grafana-opentelemetry-dotnet/actions/workflows/unit-tests.yml/badge.svg?branch=main)](https://github.com/grafana/grafana-opentelemetry-dotnet/actions/workflows/unit-tests.yml)
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-[![Nuget](https://img.shields.io/nuget/v/Grafana.OpenTelemetry.svg)](https://www.nuget.org/profiles/Grafana)
+[![NuGet](https://img.shields.io/nuget/v/Grafana.OpenTelemetry?logo=nuget&label=NuGet&color=blue)](https://www.nuget.org/profiles/Grafana)
 [![SDK](https://img.shields.io/badge/otel--sdk-1.9.0-blue?style=flat&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-dotnet)
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