Add link to Ruby docs and apply markdown formatting

ruby/use-slice-to-reorder-hash-keys.md

@@ -4,7 +4,9 @@ I recently ran into some code that was building up a hash keyed by _category_.
 It was a series of chained method calls building and transforming to produce the
 hash. It then ended with `.slice(*CATEGORIES)`.
 
-That `#slice` call at the end was doing two things:
+That
+[`Hash#slice`](https://docs.ruby-lang.org/en/master/Hash.html#method-i-slice)
+call at the end was doing two things:
 
 1. It was removing any key-value pairs not in `CATEGORIES`
 2. It was reordering the hash keys based on the order they appeared in `CATEGORIES`
@@ -23,12 +25,12 @@ Let's look at a minimum example of this:
 ```
 
 - I first define `CATEGORIES` which is an (ordered) array of symbols that
-represent the categories I care about.
+  represent the categories I care about.
 - Then I manufacture a hash of items by category. This is a bunch of dummy data,
-including extra categories, meant to demonstrate what `#slice` can do.
+  including extra categories, meant to demonstrate what `#slice` can do.
 - Lastly I call `.slice(*CATEGORIES)` on this hash which both pairs it down to
-that set of categories and reorders the hash so that the keys appears in the
-same order they do in the `CATEGORIES` array.
+  that set of categories and reorders the hash so that the keys appears in the
+  same order they do in the `CATEGORIES` array.
 
 This might be useful if I've defined `CATEGORIES` to be in a specific display
 order and I'm preparing this hash for consumption in some UI layer.