Upgrade rust/crates/structopt to 0.3.18

Test: make
Change-Id: Ib5d515b0f5e9ac7fdd0f2f8e2f00ecf54dbf63ca
diff --git a/examples/rename_all.rs b/examples/rename_all.rs
index 35f3c4f..c7c3538 100644
--- a/examples/rename_all.rs
+++ b/examples/rename_all.rs
@@ -17,6 +17,10 @@
 //! - **Snake Case**: Keep all letters lowercase and indicate word boundaries
 //!                   with underscores.
 //! - **Verbatim**: Use the original attribute name defined in the code.
+//!
+//! - **Lower Case**: Keep all letters lowercase and remove word boundaries.
+//!
+//! - **Upper Case**: Keep all letters upperrcase and remove word boundaries.
 
 use structopt::StructOpt;