Improve pruneMode.String()

Current version is from E2. Need change it to `--prune.mode=archive/full/minimal`

Now i see:
```
EROR[02-28|14:44:15.467] Erigon startup                           err="not allowed change of --prune flag, last time you used: --prune.h.older=100000"
not allowed change of --prune flag, last time you used: --prune.h.older=100000
```

