CH02::pluralization: update to follow standard conventions

Existing text used "bitcoin" as both the singular and plural unit, which
was not an uncommon practice when the first edition was written.
However, my edits to the first chapter and this update adopt the
practice of using the same pluralization rules used for other
currencies, like the dollar, plus most numbered things in general.
This produces more natural text.

Reference: https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Numbers/faq0058.html

ch02.asciidoc

@@ -66,9 +66,9 @@ you've received, spent, and currently own.
 ==== Buying from an Online Store
 
 Alice, introduced in the previous chapter, is a new user who has just
-acquired her first bitcoin. In <<getting_first_bitcoin>>, Alice met with
-her friend Joe to exchange some cash for bitcoin. The transaction
-created by Joe funded Alice's wallet with 0.10 BTC. Now Alice will make
+acquired her first bitcoins. In <<getting_first_bitcoin>>, Alice met with
+her friend Joe to exchange some cash for bitcoins. Since then, Alice has
+bought additional bitcoins.  Now Alice will make
 her first retail transaction, buying a laptop from Bob's online store.
 
 Bob's web store recently started accepting bitcoin payments by adding a
@@ -145,10 +145,12 @@ can spend that amount in subsequent transactions.
 ((("fractional values")))((("milli-bitcoin")))((("satoshis")))The
 Bitcoin network can transact in fractional values, e.g., from
 millibitcoin (1/1000th of a bitcoin) down to 1/100,000,000th of a
-bitcoin, which is known as a satoshi.  Throughout this book, we’ll use
-the term “bitcoin” to refer to any quantity of bitcoin currency, from
-the smallest unit (1 satoshi) to the total number (21,000,000) of all
-bitcoin that will ever be mined.
+bitcoin, which is known as a satoshi.  This book uses the same
+pluralization rules used for dollars and other traditional currencies
+when talking about amounts greater than one bitcoin and when using
+decimal notation, such as "10 bitcoins" or "0.001 bitcoins."  The same
+rules also apply to other bitcoin bookkeeping units, such as
+millibitcoins and satoshis.
 ====
 
 You can examine Alice's transaction to Bob's Store on the blockchain