Fix a typo (rust-lang#477)

Author: CreepySkeleton

src/walkthrough.md

@@ -47,7 +47,7 @@ before, not all of these are needed for every type of contribution.
   audience. You can find the original discussion [here][prerfc].
 - **RFC**  This is when you formally present your idea to the community for
   consideration. You can find the RFC [here][rfc].
-- **Implementation** Implement your idea unstabley in the compiler. You can
+- **Implementation** Implement your idea unstably in the compiler. You can
   find the original implementation [here][impl1].
 - **Possibly iterate/refine** As the community gets experience with your
   feature on the nightly compiler and in `libstd`, there may be additional