Rename "Create a simple exporter" -> "Create instrumented application"

This isn't an exporter. In Prometheus terms an exporter is something that exports metrics from an 3rd party to prometheus: https://prometheus.io/docs/instrumenting/writing_exporters/#writing-exporters

Author: discordianfish

README.md

@@ -252,7 +252,7 @@ Summary is similar to histogram and calculates quantiles which can be configured
 
 [Source](https://www.youtube.com/watch?v=nJMRmhbY5hY)
 
-# Create a simple exporter
+# Create instrumented application
 
 We will create a request counter metric exporter using Go.