Fix Belt doc layout, small typos (rescript-lang#29)

* Regroup HashMap, HashSet and their families as mutable collection

* revert regrouping

* Correct description of Belt.Map.update

Author: namenu

pages/apis/latest/belt/map-dict.mdx

@@ -2,7 +2,7 @@
 
 <Intro>
 
-This module seprate identity from data, it is a bit more verboe but slightly more efficient due to the fact that there is no need to pack identity and data back after each operation
+This module separates identity from data, it is a bit more verbose but slightly more efficient due to the fact that there is no need to pack identity and data back after each operation
 
 **_Advanced usage only_**
 

pages/apis/latest/belt/map.mdx

@@ -514,7 +514,7 @@ let updateU: (t<'k, 'v, 'id>, 'k, (. option<'v>) => option<'v>) => t<'k, 'v, 'id
 let update: (t<'k, 'v, 'id>, 'k, option<'v> => option<'v>) => t<'k, 'v, 'id>
 ```
 
-`update(m, x, f)` returns a map containing the same bindings as `m`, except for the binding of `x`. Depending on the value of `y` where `y` is `f(get(x, m))`, the binding of `x` is added, removed or updated. If `y` is `None`, the binding is removed if it exists; otherwise, if `y` is `Some(z)` then `x` is associated to `z` in the resulting map.
+`update(m, x, f)` returns a map containing the same bindings as `m`, except for the binding of `x`. Depending on the value of `y` where `y` is `f(get(m, x))`, the binding of `x` is added, removed or updated. If `y` is `None`, the binding is removed if it exists; otherwise, if `y` is `Some(z)` then `x` is associated to `z` in the resulting map.
 
 ## mergeMany
 

pages/apis/latest/belt/set-dict.mdx

@@ -2,7 +2,7 @@
 
 <Intro>
 
-This module seprate identity from data, it is a bit more verboe but slightly more efficient due to the fact that there is no need to pack identity and data back after each operation
+This module separates identity from data, it is a bit more verbose but slightly more efficient due to the fact that there is no need to pack identity and data back after each operation
 
 </Intro>