Use divs with ids rather than as with names.

The id attribute has been an official part of HTML since 1997. There is no
reason not to use it.

src/doc/trpl/error-handling.md

@@ -87,7 +87,7 @@ thread '<main>' panicked at 'Invalid number: 11', src/bin/panic-simple.rs:5
 Here's another example that is slightly less contrived. A program that accepts
 an integer as an argument, doubles it and prints it.
 
-<a name="code-unwrap-double"/>
+<div id="code-unwrap-double">
 ```rust,should_panic
 
 use std::env;
@@ -99,6 +99,7 @@ fn main() {
     println!("{}", 2 * n);
 }
 ```
+</div>
 
 If you give this program zero arguments (error 1) or if the first argument
 isn't an integer (error 2), the program will panic just like in the first
@@ -139,7 +140,7 @@ system is an important concept because it will cause the compiler to force the
 programmer to handle that absence. Let's take a look at an example that tries
 to find a character in a string:
 
-<a name="code-option-ex-string-find"/>
+<div id="code-option-ex-string-find">
 ```rust
 // Searches `haystack` for the Unicode character `needle`. If one is found, the
 // byte offset of the character is returned. Otherwise, `None` is returned.
@@ -152,6 +153,7 @@ fn find(haystack: &str, needle: char) -> Option<usize> {
     None
 }
 ```
+</div>
 
 Notice that when this function finds a matching character, it doen't just
 return the `offset`. Instead, it returns `Some(offset)`. `Some` is a variant or