small fix on first dplyr lesson

Getting_and_Cleaning_Data/Manipulating_Data_with_dplyr/lesson.yaml

@@ -121,7 +121,7 @@
   Hint: See what -5:20 gives us.
 
 - Class: cmd_question
-  Output: Oops! That gaves us a vector of numbers from -5 through 20, which is not what we want. Instead, we want to negate the entire sequence of numbers from 5 through 20, so that we get -5, -6, -7, ... , -18, -19, -20. Try the same thing, except surround 5:10 with parentheses so that R knows we want it to first come up with the sequence of numbers, then apply the negative sign to the whole thing.
+  Output: Oops! That gaves us a vector of numbers from -5 through 20, which is not what we want. Instead, we want to negate the entire sequence of numbers from 5 through 20, so that we get -5, -6, -7, ... , -18, -19, -20. Try the same thing, except surround 5:20 with parentheses so that R knows we want it to first come up with the sequence of numbers, then apply the negative sign to the whole thing.
   CorrectAnswer: -(5:20)
   AnswerTests: omnitest(correctExpr='-(5:20)')
   Hint: -(5:20) will do the job.