Incorporate dyn into more comments and docs.

src/libstd/fs.rs

@@ -256,7 +256,7 @@ fn initial_buffer_size(file: &File) -> usize {
 /// use std::fs;
 /// use std::net::SocketAddr;
 ///
-/// fn main() -> Result<(), Box<std::error::Error + 'static>> {
+/// fn main() -> Result<(), Box<dyn std::error::Error + 'static>> {
 ///     let foo: SocketAddr = String::from_utf8_lossy(&fs::read("address.txt")?).parse()?;
 ///     Ok(())
 /// }
@@ -298,7 +298,7 @@ pub fn read<P: AsRef<Path>>(path: P) -> io::Result<Vec<u8>> {
 /// use std::fs;
 /// use std::net::SocketAddr;
 ///
-/// fn main() -> Result<(), Box<std::error::Error + 'static>> {
+/// fn main() -> Result<(), Box<dyn std::error::Error + 'static>> {
 ///     let foo: SocketAddr = fs::read_to_string("address.txt")?.parse()?;
 ///     Ok(())
 /// }