DOC: suppress plot display in doctests

Plot display interrupts the doctest run.

skimage/transform/finite_radon_transform.py

@@ -52,7 +52,7 @@ def frt2(a):
     >>> plt.imshow(f, interpolation='nearest', cmap=plt.cm.gray)
     >>> plt.xlabel('Angle')
     >>> plt.ylabel('Translation')
-    >>> plt.show()
+    >>> # plt.show()
 
     References
     ----------

skimage/transform/hough_transform.py

@@ -137,7 +137,7 @@ def hough(img, theta=None):
     >>> plt.imshow(out, cmap=plt.cm.bone)
     >>> plt.xlabel('Angle (degree)')
     >>> plt.ylabel('Distance %d (pixel)' % d[0])
-    >>> plt.show()
+    >>> # plt.show()
 
     .. plot:: hough_tf.py