03-loop.md: fix 'Computing the Value of a Polynomial' exercise (#757)

The Output block in this exercise contains two assignments to the variable `y`, which neither looks like an Output nor does it follow from the preceding Python block when run.

I believe the first assignment to `y` was intended to be part of the Python block, and I also believe the intent was to print the value of `y`, which is 97, instead of assigning 97 to it in the Output block.

This commit does the following:
- moves the first `y` assignment from the Output block into the Python block
- adds a `print(y)` statement to the end of the Python block
- removes the assignment from the `y = 97` line, so that the Output block matches the output of the edited Python block when it is run.

_episodes/03-loop.md

@@ -390,12 +390,13 @@ so we should always use it when we can.
 > ~~~
 > x = 5
 > coefs = [2, 4, 3]
+> y = coefs[0] * x**0 + coefs[1] * x**1 + coefs[2] * x**2
+> print(y)
 > ~~~
 > {: .language-python}
 >
 > ~~~
-> y = coefs[0] * x**0 + coefs[1] * x**1 + coefs[2] * x**2
-> y = 97
+> 97
 > ~~~
 > {: .output}
 >