Merge pull request #2218 from kjacks21/mssim-float-fix

[MRG] DOC: corrected mssim docstring to return float

skimage/measure/_structural_similarity.py

@@ -54,7 +54,7 @@ def compare_ssim(X, Y, win_size=None, gradient=False,
 
     Returns
     -------
-    mssim : float or ndarray
+    mssim : float
         The mean structural similarity over the image.
     grad : ndarray
         The gradient of the structural similarity index between X and Y [2]_.