fix: mentioned 'fract' instead of 'rand'

[WojtekPachowiak]

we're interpolating between floored points to which rand is additionally applied - not fract.

See also code excerpt:

    // Four corners in 2D of a tile
    float a = random(i);
    float b = random(i + vec2(1.0, 0.0));
    float c = random(i + vec2(0.0, 1.0));
    float d = random(i + vec2(1.0, 1.0));