02-numpy.md: wrap 'tab' in <kbd> HTML tag (#850)

_episodes/02-numpy.md

@@ -386,7 +386,7 @@ standard deviation: 4.61383319712
 > How did we know what functions NumPy has and how to use them?
 > If you are working in IPython or in a Jupyter Notebook, there is an easy way to find out.
 > If you type the name of something followed by a dot, then you can use tab completion
-> (e.g. type `numpy.` and then press tab)
+> (e.g. type `numpy.` and then press <kbd>Tab</kbd>)
 > to see a list of all functions and attributes that you can use. After selecting one, you
 > can also add a question mark (e.g. `numpy.cumprod?`), and IPython will return an
 > explanation of the method! This is the same as doing `help(numpy.cumprod)`.