Update 02-variables.md upper-lower case change

Thanks a lot for the great learning materials. I have one suggestion. The original statement upper-case letters are the conventions so we use lower case is ambiguous. I guess this was just mixed up, but if not this would need further explanation in my opinion.

episodes/02-variables.md

@@ -185,7 +185,7 @@ print(len('helium'))
 
 - Python thinks that upper- and lower-case letters are different,
   so `Name` and `name` are different variables.
-- There are conventions for using upper-case letters at the start of variable names so we will use lower-case letters for now.
+- There are conventions for using lower-case letters at the start of variable names so we will use lower-case letters for now.
 
 ## Use meaningful variable names.