Improve grammar and punctuation of accelerator.md

Improve the grammar, sentence structure, punctuation and formatting of
`accelerator.md`.

docs/api/accelerator.md

@@ -1,6 +1,6 @@
 # Accelerator
 
-An accelerator is string that represents a keyboard shortcut, it can contain
+An accelerator is a string that represents a keyboard shortcut. It can contain
 multiple modifiers and key codes, combined by the `+` character.
 
 Examples:
@@ -10,11 +10,11 @@ Examples:
 
 ## Platform notice
 
-On Linux and Windows, the `Command` key would not have any effect, you can
+On Linux and Windows, the `Command` key does not have any effect so
 use `CommandOrControl` which represents `Command` on OS X and `Control` on
 Linux and Windows to define some accelerators.
 
-The `Super` key is mapped to the `Windows` key on Windows and Linux and
+The `Super` key is mapped onto the `Windows` key on Windows and Linux and onto
 `Cmd` on OS X.
 
 ## Available modifiers