Fix a typo when receiving wrong idx.

Author: Jacobluke-

dabest/_classes.py

@@ -188,7 +188,7 @@ def __init__(self, data, idx, x, y, paired, id_col, ci,
                 raise ValueError(err0 + err1 + err2)
 
         else: # mix of string and tuple?
-            err = 'There seems to be a problem with the idx you'
+            err = 'There seems to be a problem with the idx you '\
             'entered--{}.'.format(idx)
             raise ValueError(err)