Upgrade rust/crates/structopt to 0.3.22

Test: make
Change-Id: Id5e1a89dea1beece1f6ec549d6afc4dac67dc9b4
diff --git a/examples/example.rs b/examples/example.rs
index 7a9a514..71cc124 100644
--- a/examples/example.rs
+++ b/examples/example.rs
@@ -37,7 +37,7 @@
 
     // An optional list of values, will be `None` if not present on
     // the command line, will be `Some(vec![])` if no argument is
-    // provided (i.e. `--optv`) and will be `Some(Some(String))` if
+    // provided (i.e. `--optv`) and will be `Some(Vec<String>)` if
     // argument list is provided (e.g. `--optv a b c`).
     #[structopt(long)]
     optv: Option<Vec<String>>,