Updates to emacs syntax highlighting instructions (#1577)

* Add highlight-numbers package requirement

* Update fuse-mode to futil-mode

* Clarify pure emacs vs spacemacs

docs/tools/editor-highlighting.md

@@ -22,15 +22,21 @@ And run:
 `futil-mode` is implements highlighting for `.futil` files in emacs.
 It is located in `<repo>/tools/emacs/futil-mode`.
 
+The `highlight-numbers` package is required as part of `futil-mode`, install it:
+```
+M-x package-install RET highlight-numbers RET
+```
+
 Clone the repository, add the above path to your [load path][], and require
-`futil-mode`.
-If you use [Spacemacs][], this looks like adding the following lines to
-`dotspacemacs/user-config` in your `.spacemacs` file:
+`futil-mode` in your `.emacs` file:
 ```elisp
-(push "~/.emacs.d/private/local/fuse-mode" load-path)
-(require 'fuse-mode)
+(push "~/.emacs.d/private/local/futil-mode" load-path)
+(require 'futil-mode)
 ```
-I imagine it looks very similar for pure emacs, but haven't actually tried it myself.
+
+If you use [Spacemacs][], you would add this to `dotspacemacs/user-config`
+in your `.spacemacs`.
+
 
 ## Visual Studio Code