Update episodes/03-skimage-images.md

Fix typo/grammar

Co-authored-by: Marianne Corvellec <marianne.corvellec@ens-lyon.org>

Author: uschille

episodes/03-skimage-images.md

@@ -65,7 +65,7 @@ ax.imshow(chair)
 
 Once we have the image in the program,
 we first call `plt.subplots()` so that we will have
-a fresh figure with a set of axis independent from our previous calls.
+a fresh figure with a set of axes independent from our previous calls.
 Next we call `ax.imshow()` in order to display the image.
 
 Now, we will save the image in another format: