change exercises_preface text to literal block

_modules/filter_morphological.md

@@ -45,16 +45,15 @@ activity_preface: >
 # activities:
 #   "Platform": "path/to/file.md"
 
-exercises_preface: >
-
+exercises_preface: |
   ### Exercise 1
 
   Fill in the blanks, using those words: "shrinks", "increases", "decreases", "enlarges".
 
-  1. An erosion _____ objects in a binary image.
-  2. An erosion in a binary image _____ the number of foreground pixels.
-  3. A dilation in a grayscale image _____ the average intensity in the image.
-  4. A dilation _____ objects in a binary image.
+  1. An erosion \_\_\_\_\_ objects in a binary image.
+  2. An erosion in a binary image \_\_\_\_\_ the number of foreground pixels.
+  3. A dilation in a grayscale image \_\_\_\_\_ the average intensity in the image.
+  4. A dilation \_\_\_\_\_ objects in a binary image.
 
 
   ### Exercise 2