Removing namespace added for testing

Changed the namespace because the script I was testing with already had
Fake added and I wasn't sure if #r-ing SlackNotification.fs into the
script would mess up if they were in the same namespace. So I changed
it and now I am deleting it.

src/app/Fake.Api.Slack/SlackNotification.fs

@@ -1,4 +1,4 @@
-namespace Fake.Api.MyApi
+namespace Fake.Api
 
 #if NETSTANDARD
 open System.Net.Http