Update forkJoin.theory.md

projects/combining-streams/src/lib/exercises/forkJoin/docs/forkJoin.theory.md

@@ -6,7 +6,7 @@ from different `Observables` and transform them into a single emission.
 The resulting value is an array with the last emitted value of all included Observables.
 
 In this very exercise, we will utilize the `forkJoin` operator. 
-On first sight, it is a perfect match for combining HTTP Requests since it waits until all
+At first sight, it is a perfect match for combining HTTP Requests since it waits until all
 combined operators `complete` before emitting a result.
 
 ## Behavior