Apply suggestions from code review

Co-authored-by: Michael Vines <mvines@gmail.com>

validator/src/main.rs

@@ -1205,11 +1205,11 @@ pub fn main() {
                 .help("Path to accounts shrink path which can hold a compacted account set."),
         )
         .arg(
-            Arg::with_name("snapshots_path")
-                .long("snapshots")
-                .value_name("PATHS")
+            Arg::with_name("snapshot_dir")
+                .long("snapshot-dir")
+                .value_name("DIR")
                 .takes_value(true)
-                .help("Snapshots location"),
+                .help("Use DIR as persistent snapshot [default: --ledger value]"),
         )
         .arg(
             Arg::with_name("gossip_port")