05-lists.md: Add alt-text to 05-lists (#803)

* Add alt-text to Episode 5

* Correct line lengths for alt-text

* Correct typo & phrasing of units of pepper

_episodes/05-lists.md

@@ -179,8 +179,10 @@ does not.
 >
 > Here is a visual example of how indexing a list of lists `x` works:
 >
-> [![The first element of a list.
-> Adapted from @hadleywickham.](../fig/indexing_lists_python.png)][hadleywickham-tweet]
+> [![x is represented as a pepper shaker containing several packets of pepper. [x[0]] is represented
+> as a pepper shaker containing a single packet of pepper. x[0] is represented as a single packet of
+> pepper. x[0][0] is represented as single grain of pepper.  Adapted 
+> from @hadleywickham.](../fig/indexing_lists_python.png)][hadleywickham-tweet]
 >
 > Using the previously declared list `x`, these would be the results of the
 > index operations shown in the image: