Correct the irb command's description

Author: st0012

README.md

@@ -713,7 +713,7 @@ The `<...>` notation means the argument.
 * `eval <expr>`
   * Evaluate `<expr>` on the current frame.
 * `irb`
-  * Invoke `irb` on the current frame.
+  * Activate and switch to `irb:rdbg` console
 
 ### Trace
 

lib/debug/session.rb

@@ -938,7 +938,7 @@ def register_default_command
       end
 
       # * `irb`
-      #   * Invoke `irb` on the current frame.
+      #   * Activate and switch to `irb:rdbg` console
       register_command 'irb' do |arg|
         if @ui.remote?
           @ui.puts "\nIRB is not supported on the remote console."