Fix example

Author: k-bx

README.md

@@ -13,9 +13,15 @@ more examples see the tests.
 
 ```haskell
 {-# LANGUAGE ExtendedDefaultRules #-}
-import Matplotlib
 
-onscreen $ contourF (\a b -> sin (degreesRadians a) + cos (degreesRadians b)) (-100) 100 (-200) 200 10
+import Graphics.Matplotlib
+
+degreesRadians a = a * pi / 180.0
+
+main :: IO ()
+main = do
+  Right _ <- onscreen $ contourF (\a b -> sin (degreesRadians a) + cos (degreesRadians b)) (-100) 100 (-200) 200 10
+  return ()
 ```
 
 We need `-XExtendedDefaultRules` to avoid having to manually having to specify certain types.