Revert naming changes to Event tracking

- It was suggested that this change was not necessary and give false
impression that it belongs to group::analylitcs when it does not.

doc/development/event_tracking/index.md

-# Product Analytics
+# Event tracking
 
 At GitLab, we encourage event tracking so we can iterate on and improve the project and user experience.
 
@@ -44,7 +44,7 @@ From the backend, the events that are tracked will likely consist of things like
 
 See [Backend tracking guide](backend.md).
 
-Also, see [Application performance metrics](../instrumentation.md) if you are after instrumenting application performance metrics.
+Also, see [Instrumenting Ruby code](../instrumentation.md) if you are instrumenting application performance metrics for Ruby code.
 
 ## Enabling tracking
 

doc/development/instrumentation.md

-# Application Performance Metrics for Ruby Code
+# Instrumenting Ruby code
 
 [GitLab Performance Monitoring](../administration/monitoring/performance/index.md) allows instrumenting of both methods and custom
 blocks of Ruby code. Method instrumentation is the primary form of
 instrumentation with block-based instrumentation only being used when we want to
 drill down to specific regions of code within a method.
 
-Please refer to [Product analytics](event_tracking/index.md) if you are after tracking product usage patterns.
+Please refer to [Event tracking](event_tracking/index.md) if you are tracking product usage patterns.
 
 ## Instrumenting Methods