README: change chainID to <arbitrary positive integer> (#20002)

* Change chainId in genesis block to 8888 from 0 

Change chainId in genesis block to 8888 from 0 per Moriteru in https://ethereum.stackexchange.com/a/28082/40230.

* Replace 8888 with “<arbitrary positive integer>”

Per PR review, replace Moriteru’s arbitrary positive integer 8888 with “<arbitrary positive integer>” in chainId field.

README.md

@@ -210,7 +210,7 @@ aware of and agree upon. This consists of a small JSON file (e.g. call it `genes
 ```json
 {
   "config": {
-    "chainId": 0,
+    "chainId": <arbitrary positive integer>,
     "homesteadBlock": 0,
     "eip150Block": 0,
     "eip155Block": 0,