README: accounts in alloc field should exist (ethereum#20005)

* Accounts in alloc field must already exist

Note that accounts in alloc field must already exist, as pointed out by Simeon Vanov in https://gettoshare.com/2017/10/30/how-to-use-genesis-json-alloc-property/

* Change wording per PR review comment

* README: minor fixups

README.md

@@ -233,8 +233,8 @@ aware of and agree upon. This consists of a small JSON file (e.g. call it `genes
 
 The above fields should be fine for most purposes, although we'd recommend changing
 the `nonce` to some random value so you prevent unknown remote nodes from being able
-to connect to you. If you'd like to pre-fund some accounts for easier testing, you can
-populate the `alloc` field with account configs:
+to connect to you. If you'd like to pre-fund some accounts for easier testing, create
+the accounts and populate the `alloc` field with their addresses.
 
 ```json
 "alloc": {