CH02::Alice pays Bob: remove text about payment completion

Existing text claimed that Alice's unconfirmed transaction completed the
payment to Bob, but Bob would be wise to wait for a $750 transaction to
confirm.

I think this is a problem resulting from a previous edit to the
second-edition text where the example was changed from a cup of coffee
at a cafe to a laptop.  A cup of coffee is cheap and has no resale
value, so the chance of fraud is small; a laptop is expensive and has
significant resale value, making fraud a greater risk.

ch02.asciidoc

@@ -127,7 +127,7 @@ Alice uses her smartphone to scan the barcode on display. Her smartphone
 shows a payment for the correct amount to +Bob's Store+ and she selects Send to
 authorize the payment. Within a few seconds (about the same amount of
 time as a credit card authorization), Bob sees the transaction on the
-register, completing the transaction.
+register.
 
 In the following sections, we will examine this transaction in more
 detail. We'll see how Alice's wallet constructed it, how it was