Update understanding_qmk.md (qmk#19664)

Changed "...4 row by 5 column numpad" to "...5 row by 4 column numpad" on line 46.

docs/understanding_qmk.md

@@ -43,7 +43,7 @@ While there are different strategies for doing the actual matrix detection, they
 }
 ```
 
-That datastructure is a direct representation of the matrix for a 4 row by 5 column numpad. When a key is pressed that key's position within the matrix will be returned as `1` instead of `0`.
+That datastructure is a direct representation of the matrix for a 5 row by 4 column numpad. When a key is pressed that key's position within the matrix will be returned as `1` instead of `0`.
 
 Matrix Scanning runs many times per second. The exact rate varies but typically it runs at least 10 times per second to avoid perceptible lag.