Update quickstart with valid function example for chain_err()

There is no write_str() in the std lib on a trait for a File.

Author: nelsonjchen

README.md

@@ -60,7 +60,7 @@ the `error_chain!` macro, and start chaining errors!
 ```rust
 fn do_error_prone_work() -> Result<()> {
     let file = try!(File::open("foo").chain_err(|| "couldn't open file"));
-    try!(file.write_str("important").chain_err(|| "couldn't write file"));
+    try!(file.write_all("important".as_bytes()).chain_err(|| "couldn't write file"));
 
     Ok(())
 }

src/lib.rs

@@ -100,7 +100,7 @@
 //! ```ignore
 //! fn do_error_prone_work() -> Result<()> {
 //!     let file = try!(File::open("foo").chain_err(|| "couldn't open file"));
-//!     try!(file.write_str("important").chain_err(|| "couldn't write file"));
+//!     try!(file.write_all("important".as_bytes()).chain_err(|| "couldn't write file"));
 //!
 //!     Ok(())
 //! }