Address review comments

_episodes/03-lists.md

@@ -111,19 +111,17 @@ does not.
 >
 > Data which can be modified in place is called [mutable]({{ page.root }}/reference/#mutable),
 > while data which cannot be modified is called [immutable]({{ page.root }}/reference/#immutable).
-> Strings and numbers are immutable.
-> This does not mean that variables with string or number values are constants,
-> but when we want to change the value of a string or number variable, we can only
+> Strings and numbers are immutable. This does not mean that variables with string or number values
+> are constants, but when we want to change the value of a string or number variable, we can only
 > 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
+> 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.
 >
-> 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!
+> 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!
 >
 > ~~~
 > salsa = ['peppers', 'onions', 'cilantro', 'tomatoes']
@@ -174,9 +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.](../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:
@@ -211,7 +208,7 @@ does not.
 > ~~~
 > {: .output}
 >
-> Thanks to [Hadley Wickham](https://twitter.com/hadleywickham/status/643381054758363136)
+> Thanks to [Hadley Wickham][hadleywickham-tweet]
 > for the image above.
 {: .callout}
 
@@ -542,4 +539,6 @@ Omitting ending index: ["sep", "oct", "nov", "dec"]
 > {: .solution}
 {: .challenge}
 
+[hadleywickham-tweet]: https://twitter.com/hadleywickham/status/643381054758363136
+
 {% include links.md %}