chore(copybuffer): format shortcut and lighten text (#11078)

plugins/copybuffer/README.md

 # `copybuffer` plugin
 
-This plugin binds the ctrl-o keyboard shortcut to a command that copies the text
-that is currently typed in the command line ($BUFFER) to the system clipboard.
+This plugin adds the <kbd>ctrl-o</kbd> keyboard shortcut to copy the current text
+in the command line to the system clipboard.
 
 This is useful if you type a command - and before you hit enter to execute it - want
 to copy it maybe so you can paste it into a script, gist or whatnot.