Fix whitespace in _axes.py error message

This fixes issue matplotlib#30285.

Author: brchristian

lib/matplotlib/axes/_axes.py

@@ -2296,8 +2296,8 @@ def _parse_bar_color_args(self, kwargs):
             facecolor = mcolors.to_rgba_array(facecolor)
         except ValueError as err:
             raise ValueError(
-                "'facecolor' or 'color' argument must be a valid color or"
-                    "sequence of colors."
+                "'facecolor' or 'color' argument must be a valid color or "
+                "sequence of colors."
             ) from err
 
         return facecolor, edgecolor