Update guide index to point to the task page

When the "threads" guides were renamed to be "tasks" guides, it looks
like this link was missed.

Here's the other relevant commit.

src/doc/index.md

@@ -58,7 +58,7 @@ a guide that can help you out:
 * [Strings](guide-strings.html)
 * [Pointers](guide-pointers.html)
 * [Crates and modules](guide-crates.html)
-* [Threads and Communication](guide-threads.html)
+* [Threads and Communication](guide-tasks.html)
 * [Error Handling](guide-error-handling.html)
 * [Foreign Function Interface](guide-ffi.html)
 * [Writing Unsafe and Low-Level Code](guide-unsafe.html)