doc: rename ambigous 'Subcmd layer' to 'UI layer'

Author: tyru

_docs/layer.md

@@ -1,10 +1,10 @@
 
 ## Layered architecture
 
-The volt commands like `volt get` which may modify lock.json, config.toml([#221](https://github.com/vim-volt/volt/issues/221)),
+The volt commands like `volt get` which may modify lock.json, config.toml,
 filesystem, are executed in several steps:
 
-1. (Subcmd layer): pass subcommand arguments, lock.json & config.toml structure
+1. (UI layer): pass subcommand arguments, lock.json & config.toml structure
    to Gateway layer
 2. (Gateway layer): Create an AST according to given information
     * This layer cannot touch filesystem, because it makes unit testing difficult
@@ -13,10 +13,10 @@ filesystem, are executed in several steps:
 Below is the dependency graph:
 
 ```
-Subcmd --> Gateway --> Usecase
+UI --> Gateway --> Usecase
 ```
 
-* Subcmd only depends Gateway
-* Gateway doesn't know Subcmd
+* UI only depends Gateway
+* Gateway doesn't know UI
 * Gateway only depends Usecase
 * Usecase doesn't know Gateway