message: Change Keymap wording for gray image in curve mode
  Instead of being a copy of line 76, line 79 instead correctly indicates that it will show a histogram for a gray image in curve mode, as given by the code block at line 103 referencing image "gray" instead of image "im".
diff: @@ -76,7 +76,7 @@ def main():
  a - show histogram for color image in curve mode \n
  b - show histogram in bin mode \n
  c - show equalized histogram (always in bin mode) \n
  - d - show histogram for color image in curve mode \n
  + d - show histogram for gray image in curve mode \n
  e - show histogram for a normalized image in curve mode \n
  Esc - exit \n
  ''')
  