doc: fix redirected link in /index.html

Author: notriddle

src/doc/index.md

@@ -101,7 +101,7 @@ accomplishing various tasks.
 
 Many of Rust's errors come with error codes, and you can request extended
 diagnostics from the compiler on those errors. You can also [read them
-here](error-index.html), if you prefer to read them that way.
+here](error_codes/index.html), if you prefer to read them that way.
 
 # Master Rust