change variable 'x' to 'x_val'

It's good to emphasise the use of variable names that are more descriptive. One letter variables may provide room for errors in longer scripts.

Author: SaymoreChifamba

_episodes/01-numpy.md

@@ -38,7 +38,7 @@ weight_kg = 55
 {: .python}
 
 A variable is just a name for a value,
-such as `x`, `current_temperature`, or `subject_id`.
+such as `x_val`, `current_temperature`, or `subject_id`.
 Python's variables must begin with a letter and are [case sensitive](reference.html#case-sensitive).
 We can create a new variable by assigning a value to it using `=`.
 When we are finished typing and press Shift+Enter,