fix typo in error message (#63)

Author: mathause

mplotutils/xrcompat.py

@@ -11,7 +11,7 @@ def infer_interval_breaks(x, y, clip=False):
 
     # TODO: require cartopy >= 0.21 when removing this function
     warnings.warn(
-        "It's no longer necessary to compute the edges of the array. This is now done"
+        "It's no longer necessary to compute the edges of the array. This is now done "
         "in matplotlib. This function will be removed from mplotutils in a future "
         "version.",
         FutureWarning,