book: Fix a hyperlink to CONFIGS.md

Right now rustdoc replaces the string ".md)" with ".html)" to fix links between
markdown files, so use a different syntax that doesn't get caught in the
crossfire.

Closes #22900

Author: alexcrichton

src/doc/trpl/hello-world.md

@@ -89,7 +89,9 @@ This line does all of the work in our little program. There are a number of
 details that are important here. The first is that it's indented with four
 spaces, not tabs. Please configure your editor of choice to insert four spaces
 with the tab key. We provide some [sample configurations for various
-editors](https://github.com/rust-lang/rust/tree/master/src/etc/CONFIGS.md).
+editors][configs].
+
+[configs]: https://github.com/rust-lang/rust/tree/master/src/etc/CONFIGS.md
 
 The second point is the `println!()` part. This is calling a Rust *macro*,
 which is how metaprogramming is done in Rust. If it were a function instead, it

src/rustbook/build.rs

@@ -89,7 +89,8 @@ fn render(book: &Book, tgt: &Path) -> CliResult<()> {
         } else {
             src = PathBuf::from(&env::args().nth(2).unwrap());
         }
-        // preprocess the markdown, rerouting markdown references to html references
+        // preprocess the markdown, rerouting markdown references to html
+        // references
         let mut markdown_data = String::new();
         try!(File::open(&src.join(&item.path)).and_then(|mut f| {
             f.read_to_string(&mut markdown_data)