Avoid the variable name 'char'

This changes a variable name from ```char``` to ```letter``` to avoid
possible confusion of learners with previous experience in other
programming languages that do have a character data type.

Author: knarrff

_episodes/05-loop.md

@@ -307,8 +307,8 @@ so we should always use it when we can.
 > Given the following loop:
 > ~~~
 > word = 'oxygen'
-> for char in word:
->     print(char)
+> for letter in word:
+>     print(letter)
 > ~~~
 > {: .language-python}
 >