Update documentation

Author: william-lawrence

README.md

@@ -675,6 +675,8 @@ The `<…>` notation means the argument.
   * Show current frame's source code from the line <start> to <end> if given.
 * `whereami`
   * Show the current frame with source code.
+* `@`
+  * Same as `whereami`.
 * `edit`
   * Open the current file on the editor (use `EDITOR` environment variable).
   * Note that edited file will not be reloaded.

lib/debug/session.rb

@@ -739,6 +739,8 @@ def register_default_command
 
       # * `whereami`
       #   * Show the current frame with source code.
+      # * `@`
+      #   * Same as `whereami`.
       register_command 'whereami', '@', unsafe: false do
         request_tc [:show, :whereami]
       end