doc: update nixpkgs-mozilla rev

The previous one didn't work, returning a TOML parse error.

doc/languages-frameworks/rust.section.md

@@ -336,9 +336,9 @@ with import <nixpkgs> {};
 let src = fetchFromGitHub {
       owner = "mozilla";
       repo = "nixpkgs-mozilla";
-      # commit from: 2018-03-27
-      rev = "2945b0b6b2fd19e7d23bac695afd65e320efcebe";
-      sha256 = "034m1dryrzh2lmjvk3c0krgip652dql46w5yfwpvh7gavd3iypyw";
+      # commit from: 2019-05-15
+      rev = "9f35c4b09fd44a77227e79ff0c1b4b6a69dff533";
+      sha256 = "18h0nvh55b5an4gmlgfbvwbyqj91bklf1zymis6lbdh75571qaz0";
    };
 in
 with import "${src.out}/rust-overlay.nix" pkgs pkgs;