fix docs

Co-authored-by: Daniel Badura <[email protected]>

docs/pages/subscription.md

@@ -1,6 +1,6 @@
 # Subscriptions
 
-A one core concept of event sourcing is the ability to react and process events in a different way.
+One core concept of event sourcing is the ability to react and process events in a different way.
 This is where subscriptions and subscription engine come into play.
 
 There are different types of subscriptions. In most cases, we are talking about projector and processor.