[ENH] Add line breaks in _episodes/03-lists

_episodes/03-lists.md

@@ -116,11 +116,11 @@ does not.
 > replace the old value with a completely new value.
 >
 > Lists and arrays, on the other hand, are mutable: we can modify them after they have been
-> created. We can change individual elements, append new elements, or reorder the whole list.  For
-> some operations, like sorting, we can choose whether to use a function that modifies the data in
-> place or a function that returns a modified copy and leaves the original unchanged.
+> created. We can change individual elements, append new elements, or reorder the whole list. For
+> some operations, like sorting, we can choose whether to use a function that modifies the data
+> in-place or a function that returns a modified copy and leaves the original unchanged.
 >
-> Be careful when modifying data in place.  If two variables refer to the same list, and you modify
+> Be careful when modifying data in-place. If two variables refer to the same list, and you modify
 > the list value, it will change for both variables!
 >
 > ~~~
@@ -172,8 +172,8 @@ does not.
 >
 > Here is a visual example of how indexing a list of lists `x` works:
 >
-> <a href='https://twitter.com/hadleywickham/status/643381054758363136'>
-> ![The first element of a list. Adapted from @hadleywickham's tweet about R lists.](../fig/indexing_lists_python.png)</a>
+> [![The first element of a list.
+> 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:
@@ -208,13 +208,13 @@ does not.
 > ~~~
 > {: .output}
 >
-> Thanks to [Hadley Wickham](https://twitter.com/hadleywickham/status/643381054758363136)
+> Thanks to [Hadley Wickham][hadleywickham-tweet]
 > for the image above.
 {: .callout}
 
 > ## Heterogeneous Lists
 > Lists in Python can contain elements of different types. Example:
-> ~~~ 
+> ~~~
 > sample_ages = [10, 12.5, 'Unknown']
 > ~~~
 {: .callout}
@@ -539,4 +539,6 @@ Omitting ending index: ["sep", "oct", "nov", "dec"]
 > {: .solution}
 {: .challenge}
 
+[hadleywickham-tweet]: https://twitter.com/hadleywickham/status/643381054758363136
+
 {% include links.md %}