Replace ok().expect() by Result::expect in trait chapter of trpl

Author: fhartwig

src/doc/trpl/traits.md

@@ -247,7 +247,7 @@ won’t have its methods:
 [write]: ../std/io/trait.Write.html
 
 ```rust,ignore
-let mut f = std::fs::File::open("foo.txt").ok().expect("Couldn’t open foo.txt");
+let mut f = std::fs::File::open("foo.txt").expect("Couldn’t open foo.txt");
 let buf = b"whatever"; // byte string literal. buf: &[u8; 8]
 let result = f.write(buf);
 # result.unwrap(); // ignore the error
@@ -266,7 +266,7 @@ We need to `use` the `Write` trait first:
 ```rust,ignore
 use std::io::Write;
 
-let mut f = std::fs::File::open("foo.txt").ok().expect("Couldn’t open foo.txt");
+let mut f = std::fs::File::open("foo.txt").expect("Couldn’t open foo.txt");
 let buf = b"whatever";
 let result = f.write(buf);
 # result.unwrap(); // ignore the error