Fix on-stack dispatch example (#137)

A `char` can't be used as a path. You already use the correct `"-"` in the "Disadvantages" example.
Remove `ignore` for the example block

idioms/on-stack-dyn-dispatch.md

@@ -9,14 +9,18 @@ below:
 
 ## Example
 
-```rust,ignore
-std::io::File;
+```rust
+use std::io;
+use std::fs;
+
+# fn main() -> Result<(), Box<dyn std::error::Error>> {
+# let arg = "-";
 
 // These must live longer than `readable`, and thus are declared first:
 let (mut stdin_read, mut file_read);
 
 // We need to ascribe the type to get dynamic dispatch.
-let readable: &mut dyn io::Read = if arg == '-' {
+let readable: &mut dyn io::Read = if arg == "-" {
     stdin_read = io::stdin();
     &mut stdin_read
 } else {
@@ -25,6 +29,9 @@ let readable: &mut dyn io::Read = if arg == '-' {
 };
 
 // Read from `readable` here.
+
+# Ok(())
+# }
 ```
 
 ## Motivation