Fixing a few typos.

Rx.NET/Source/System.Reactive.Interfaces/IObservable.cs

@@ -18,7 +18,7 @@ namespace System
         IDisposable Subscribe(IObserver<T> observer);
     }
 }
-#elif !WINDOWSPHONE7 // TypeForwardedTo is not presen on windows phone 7 so we can't really target
+#elif !WINDOWSPHONE7 // TypeForwardedTo is not present on Windows Phone 7 so we can't really target
 [assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.IObservable<>))]
 #endif
 

Rx.NET/Source/System.Reactive.Interfaces/IObserver.cs

@@ -30,6 +30,6 @@ namespace System
         void OnCompleted();
     }
 }
-#elif !WINDOWSPHONE7 // TypeForwardedTo is not presen on windows phone 7 so we can't really target
+#elif !WINDOWSPHONE7 // TypeForwardedTo is not present on Windows Phone 7 so we can't really target
 [assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.IObserver<>))]
 #endif