Add playground to en_US

Author: sergeysova

locales/en-US/common.ftl

@@ -24,6 +24,7 @@ nav-tools = Tools
 nav-governance = Governance
 nav-community = Community
 nav-blog = Blog
+nav-playground = Playground
 
 choose-language = Language
 
@@ -54,4 +55,4 @@ footer-attribution = Maintained by the Rust Team. See a typo?
 footer-old-site = Looking for the <a href="https://prev.rust-lang.org">previous website</a>?
 
 ## what/index.hbs
-what-header = What
+what-header = What