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".

samples/python/hist.py

@@ -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
     ''')