Fix code in WASI-tutorial.md

docs/WASI-tutorial.md

@@ -121,14 +121,14 @@ use std::io::{Read, Write};
 
 fn process(input_fname: &str, output_fname: &str) -> Result<(), String> {
     let mut input_file =
-        fs::File::open(input_fname).map_err(|err| format!("error opening input: {}", err))?;
+        fs::File::open(input_fname).map_err(|err| format!("error opening input {}: {}", input_fname, err))?;
     let mut contents = Vec::new();
     input_file
         .read_to_end(&mut contents)
         .map_err(|err| format!("read error: {}", err))?;
 
     let mut output_file = fs::File::create(output_fname)
-        .map_err(|err| format!("error opening output '{}': {}", output_fname, err))?;
+        .map_err(|err| format!("error opening output {}: {}", output_fname, err))?;
     output_file
         .write_all(&contents)
         .map_err(|err| format!("write error: {}", err))
@@ -139,7 +139,7 @@ fn main() {
     let program = args[0].clone();
 
     if args.len() < 3 {
-        eprintln!("{} <input_file> <output_file>", program);
+        eprintln!("usage: {} <input_file> <output_file>", program);
         return;
     }