Added a missing word in fine-grained-reactivity.mdx (#942)

Author: petercockerell-brt

src/routes/advanced-concepts/fine-grained-reactivity.mdx

@@ -73,7 +73,7 @@ The effect takes a function that is called whenever _any_ of the signals it reli
 To grasp the concept of reactivity, it is often helpful to construct a reactive system from scratch.
 
 The following example will follow the observer pattern, where data entities (signals) will maintain a list of their subscribers (effects).
-This is a way to notify subscribers whenever a signal they changes.
+This is a way to notify subscribers whenever a signal they observe changes.
 
 Here is a basic code outline to begin: