Fix documentation of deprecated --syncmode -> --snapshots (#4376)

README.md

@@ -71,7 +71,7 @@ make erigon
 ./build/bin/erigon
 ```
 
-Default `--syncmode=snap` for `mainnet`, `goerli`, `bsc`. Other networks now have default `--syncmode=full`. Increase download speed by flag `--torrent.download.rate=20mb`. <code>🔬 See [Downloader docs](./cmd/downloader/readme.md)</code> 
+Default `--snapshots=true` for `mainnet`, `goerli`, `bsc`. Other networks now have default `--snapshots=false`. Increase download speed by flag `--torrent.download.rate=20mb`. <code>🔬 See [Downloader docs](./cmd/downloader/readme.md)</code> 
 
 Use `--datadir` to choose where to store data.