message: Update observable.md
  correcting a grammatical error
diff: @@ -323,7 +323,7 @@ There are three types of values an Observable Execution can deliver:
  - "Error" notification: sends a JavaScript Error or exception.
  - "Complete" notification: does not send a value.
  -"Next" notifications are the most important and most common type: they represent actual data being delivered to an subscriber. "Error" and "Complete" notifications may happen only once during the Observable Execution, and there can only be either one of them.
  +"Next" notifications are the most important and most common type: they represent actual data being delivered to a subscriber. "Error" and "Complete" notifications may happen only once during the Observable Execution, and there can only be either one of them.
  These constraints are expressed best in the so-called *Observable Grammar* or *Contract*, written as a regular expression:
  