Fixing the formula in the documentation of reprojectImageTo3D

fixes #25082

The documentation states correctly:

Reprojects a disparity image to 3D space.

and

The function transforms a single-channel disparity map to a 3-channel image representing a 3D surface. That is, for each pixel (x,y) and the corresponding disparity d=disparity(x,y) , it computes:

But then there is an error in the formula:

This does not make sense since the function wants to calculate $Z$, it makes no sense to have $z$ on the right side of the equation. It does not have any meaning in a disparity image. The value should be one at that coordinate, to reflect that this is a normalized homogenous coordinate in 3 dimensions.

I fixed it to

which corresponds to the implementation which is (click permalink to get to file):

Vec4d homg_pt = _Q*Vec4d(x, y, d, 1.0);

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Do not apply since this is only a documentation fix:

• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
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