Replace hoedown with pull in rustdoc

Author: GuillaumeGomez

src/Cargo.lock

@@ -26,6 +26,7 @@ rustc_trans = { path = "../librustc_trans" }
 serialize = { path = "../libserialize" }
 syntax = { path = "../libsyntax" }
 syntax_pos = { path = "../libsyntax_pos" }
+pulldown-cmark = "0.0.8"
 
 [build-dependencies]
 build_helper = { path = "../build_helper" }