Update Strings exercises solution for 3

The third exercise in the Strings chapter provided a solution that did not pass the tests. The previous RST version passed the tests, so I took most of the changes from there. I updated it a little bit so that the spacing of the printed message of the solution matches exactly with the presented message in the problem description. I also updated the problem description to mention that the number of 'e' characters that were found should be returned since the tests expect that behavior.

Author: kklamberty

pretext/Strings/Exercises.ptx

@@ -103,7 +103,7 @@ myTests().main()
             <p>Assign to a variable in your program a triple-quoted string that contains
                             your favorite paragraph of text - perhaps a poem, a speech, instructions
                             to bake a cake, some inspirational verses, etc.</p>
-            <p>Write a function that counts the number of alphabetic characters (a through z, or A through Z) in your text and then keeps track of how many are the letter &#x2018;e'.  Your function should print an analysis of the text like this:</p>
+            <p>Write a function that counts the number of alphabetic characters (a through z, or A through Z) in your text and then keeps track of (and returns) how many are the letter &#x2018;e'.  Your function should print an analysis of the text like this:</p>
             <pre>Your text contains 243 alphabetic characters, of which 109 (44.8%) are 'e'.</pre>
           </statement>
           <program interactive="activecode" language="python" xml:id="ex_8_3_editor">
@@ -144,9 +144,12 @@ def count(p):
             if achar == 'e':
                 numberOfe = numberOfe + 1
 
-    percent_with_e = (numberOfe / totalChars) * 100
-    print("Your text contains", totalChars, "alphabetic characters of which", numberOfe, "(", percent_with_e, "%)", "are 'e'.")
-
+    if totalChars != 0:
+        percent_with_e = (numberOfe / totalChars) * 100
+        print("Your text contains", totalChars, "alphabetic characters of which", numberOfe, "(" + str(percent_with_e) + "%)", "are 'e'.")
+    else:
+        print("There were no characters in the input string p")
+    return (numberOfe)
 
 p = '''
 "If the automobile had followed the same development cycle as the computer, a