fix style README.rst

pos_keyboard/README.rst

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+******************************
 USB keyboard in Point of Sale.
+******************************
 Module to connect an additional usb keyboard for Point of Sale.
----------------------------------------------------------------
+================================================================
 The processed keys:
+----------------------------
 number: 0-9
+............................
 mode: 
+............................
  qty: '/'* or "q"**
  disc: '-'* or 'd'**
  price: '*'* or 'p'**
+............................
 dot: '.'
+............................
 backspace: 'backspace'
+............................
 +/-: '+'*  or 's'**
-----------------------------
+............................
 * key on additional keyboard
 ** key on basic keyboard
+============================