fix typo in error message in plot.py (#4188)

Author: yohai

xarray/plot/plot.py

@@ -62,7 +62,7 @@ def _infer_line_data(darray, x, y, hue):
 
     else:
         if x is None and y is None and hue is None:
-            raise ValueError("For 2D inputs, please" "specify either hue, x or y.")
+            raise ValueError("For 2D inputs, please specify either hue, x or y.")
 
         if y is None:
             xname, huename = _infer_xy_labels(darray=darray, x=x, y=hue)