Merge pull request #499 from Yuki-Inoue/fix-error-message

fix typo at error message

Author: Shekharrajak

lib/daru/dataframe.rb

@@ -3096,7 +3096,7 @@ def coerce_positions *positions, size
         when Range
           size.times.to_a[positions.first]
         else
-          raise ArgumentError, 'Unkown position type.'
+          raise ArgumentError, 'Unknown position type.'
         end
       else
         positions