fix: let cannot be used for global variables (#1077)

src/transport/smtp/async_transport.rs

@@ -234,7 +234,7 @@ where
     /// a proper URL encoder, like the following cargo script:
     ///
     /// ```rust
-    /// # let _ = r#"
+    /// # const TOML: &str = r#"
     /// #!/usr/bin/env cargo
     ///
     /// //! ```cargo

src/transport/smtp/transport.rs

@@ -177,7 +177,7 @@ impl SmtpTransport {
     /// a proper URL encoder, like the following cargo script:
     ///
     /// ```rust
-    /// # let _ = r#"
+    /// # const TOML: &str = r#"
     /// #!/usr/bin/env cargo
     ///
     /// //! ```cargo