CH02::invoice: replace "payment request" with "invoice"

The payment request language is confusing because BIP70 payment requests
look a lot like the URI used here but this is not a BIP70 payment
request (and BIP70 is dead, so we need not describe it).

Instead, update the text to use the more generic word "invoice", which
has the advantage of also being simpler English.

ch02.asciidoc

@@ -88,12 +88,12 @@ $750 USD
 ----
 
 ((("payment requests")))((("QR codes", "payment requests")))Bob's
-e-commerce system will automatically create a QR code containing a
-_payment request_ (see <<payment-request-QR>>).
+e-commerce system will automatically create a QR code containing an
+_invoice_ (see <<invoice-QR>>).
 
-Unlike a QR code that simply contains a destination Bitcoin address, a
-payment request is a QR-encoded URL that contains a destination address,
-a payment amount, and a generic description such as "Bob's Store - Order
+Unlike a QR code that simply contains a destination Bitcoin address, this
+invoice is a QR-encoded URI that contains a destination address,
+a payment amount, and a description such as "Bob's Store - Order
 512". This allows a bitcoin wallet application to prefill the
 information used to send the payment while showing a human-readable
 description to the user. You can scan the QR code with a bitcoin wallet
@@ -103,8 +103,8 @@ application to see what Alice would see.
 TODO: Replace QR code with test-BTC address
 ////
 
-[[payment-request-QR]]
-.Payment request QR code
+[[invoice-QR]]
+.Invoice QR code
 image::images/mbc2_0202.png["payment-request"]
 
 [TIP]
@@ -114,14 +114,14 @@ and cautions")))((("warnings and cautions", "avoid sending money to
 addresses appearing in book")))Try to scan this with your wallet to see
 the address and amount but DO NOT SEND MONEY.
 ====
-[[payment-request-URL]]
-.The payment request QR code encodes the following URL, defined in BIP-21:
+[[invoice-URI]]
+.The invoice QR code encodes the following URI, defined in BIP21:
 ----
 bitcoin:bc1qk2g6u8p4qm2s2lh3gts5cpt2mrv5skcuu7u3e4?amount=0.01577764&
 label=Bob%27s%20Store&
 message=Purchase%20at%20Bob%27s%20Store
 
-Components of the URL
+Components of the URI
 
 A Bitcoin address: "bc1qk2g6u8p4qm2s2lh3gts5cpt2mrv5skcuu7u3e4"
 The payment amount: "0.01577764"