grains approaches: fix incorrect exponent (#1619)

Fix the wrong power of two that gives us 8.

Was `2 to the power of 4 = 8`
Changed to `2 to the power of 3 = 8`

Author: petertseng

exercises/practice/grains/.approaches/introduction.md

@@ -18,7 +18,7 @@ You can see that the exponent, or power, that `2` is raised by is always one les
 |       1 |          0 | 2 to the power of 0 = 1 |
 |       2 |          1 | 2 to the power of 1 = 2 |
 |       3 |          2 | 2 to the power of 2 = 4 |
-|       4 |          3 | 2 to the power of 4 = 8 |
+|       4 |          3 | 2 to the power of 3 = 8 |
 
 ## Approach: `pow`