fixed inconsistent indentation in book example

looks like a case of mixed tabs and spaces

src/doc/book/error-handling.md

@@ -1643,7 +1643,7 @@ fn main() {
 
     let matches = match opts.parse(&args[1..]) {
         Ok(m)  => { m }
-		Err(e) => { panic!(e.to_string()) }
+        Err(e) => { panic!(e.to_string()) }
     };
 
     if matches.opt_present("h") {