docs: hwmon: Use file modes explicitly

As stated in the mainling list[1] and by checkpatch output, the file modes
for is_visible_func should be returned explicitly. Change that in the
documentation to avoid confusion.

[1]https://lore.kernel.org/linux-hwmon/506a6e7f-4566-2dcf-37f3-0f41f4ce983b@roeck-us.net/Signed-off-by: Joaquín Ignacio Aramendía <samsagax@gmail.com>
Link: https://lore.kernel.org/r/20221212141245.52935-1-samsagax@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/hwmon-kernel-api.rst

@@ -299,7 +299,7 @@ Parameters:
 
 Return value:
 	The file mode for this attribute. Typically, this will be 0 (the
-	attribute will not be created), S_IRUGO, or 'S_IRUGO | S_IWUSR'.
+	attribute will not be created), 0444, or 0644.
 
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