filter out S-blocked issues in the "Getting Started" search (rust-lang#1687)

Author: jyn514

src/doc/rustc-dev-guide/src/getting-started.md

@@ -90,7 +90,7 @@ filtering the search to areas you're interested in. For example:
 Not all important or beginner work has issue labels.
 See below for how to find work that isn't labelled.
 
-[help-wanted-search]: https://github.com/issues?q=is%3Aopen+is%3Aissue+org%3Arust-lang+no%3Aassignee+label%3AE-easy%2C%22good+first+issue%22%2Cgood-first-issue%2CE-medium%2CE-help-wanted%2CE-mentor
+[help-wanted-search]: https://github.com/issues?q=is%3Aopen+is%3Aissue+org%3Arust-lang+no%3Aassignee+label%3AE-easy%2C%22good+first+issue%22%2Cgood-first-issue%2CE-medium%2CE-help-wanted%2CE-mentor+-label%3AS-blocked+
 [Triage]: ./contributing.md#issue-triage
 
 ### Recurring work