Update counting.mdx (#29352)

Typo in client count references.

website/content/docs/concepts/client-count/counting.mdx

@@ -83,7 +83,7 @@ Given `CM` as the number of clients for the current month and `BP` as the number
 of clients in the billing period, we found that the approximation becomes
 increasingly imprecise as:
 
-- the difference between `BC` and `CM` increases
+- the difference between `BP` and `CM` increases
 - the value of `CM` approaches zero.
 - the number of months in the billing period increase.