Update structopt to 0.3.26

Test: cd external/rust/crates && atest --host -c
Change-Id: Iaeee8c2cf42b73912e007f71bdcaeb1db91db18c
diff --git a/examples/enum_tuple.rs b/examples/enum_tuple.rs
index 0bad2e6..a88adc2 100644
--- a/examples/enum_tuple.rs
+++ b/examples/enum_tuple.rs
@@ -1,4 +1,20 @@
 //! How to extract subcommands' args into external structs.
+//!
+//! Running this example with --help prints this message:
+//! -----------------------------------------------------
+//! classify 0.3.25
+//!
+//! USAGE:
+//!     enum_tuple <SUBCOMMAND>
+//!
+//! FLAGS:
+//!     -h, --help       Prints help information
+//!     -V, --version    Prints version information
+//!
+//! SUBCOMMANDS:
+//!     foo
+//!     help    Prints this message or the help of the given subcommand(s)
+//! -----------------------------------------------------
 
 use structopt::StructOpt;