Typo in osi_common.proto in the message "Greyscale"

[PhRosenberger]

Describe the bug

As we go for American English, there is a typo in osi_common.proto in the message "Greyscale".
The bug was found during our review on release 3.7.0 in #774.

//
// \brief The description of a color within available color spaces.
//
// ColorDescription represents the visual, non-semantic appearance of an object, structure or feature within various available color spaces.
//
// Depending on the context, this may define the color of an object or structure a priori (e.g. GroundTruth objects)
// or describe a perceived color (e.g. CameraDetections).
//
message ColorDescription
{
    // Grayscale color model
    //
    optional ColorGrey grey = 1;

    // RGB (Red, Green, Blue) color model
    //
    optional ColorRGB rgb = 2;

    // RGBIR (Red, Green, Blue, Infrared) color model
    //
    optional ColorRGBIR rgbir = 3;

    // HSV (Hue, Saturation, Value) color model
    //
    optional ColorHSV hsv = 4;

    // LUV (Luminance, U-coordinate, V-coordinate) color model
    //
    optional ColorLUV luv = 5;

    // CMYK (Cyan, Magenta, Yellow, Key) color model
    //
    optional ColorCMYK cmyk = 6;
}

//
// \brief Grayscale color model
//
// ColorGrey defines a grayscale.
//
message ColorGrey
{
    // Definition of a grayscale
    //
    // Range: [0,1]
    //
    optional double grey = 1;
}

I propose to use this issue as a reminder and fix this in a major release with braking changes, as in 4.0.0