README: fix default sync mode (#23282)

README.md

@@ -64,7 +64,7 @@ $ geth console
 ```
 
 This command will:
- * Start `geth` in fast sync mode (default, can be changed with the `--syncmode` flag),
+ * Start `geth` in snap sync mode (default, can be changed with the `--syncmode` flag),
    causing it to download more data in exchange for avoiding processing the entire history
    of the Ethereum network, which is very CPU intensive.
  * Start up `geth`'s built-in interactive [JavaScript console](https://geth.ethereum.org/docs/interface/javascript-console),