doc(tutorial_py_fourier_transform): Correct errors in tutorial for integer division and filter symmetry

Found via `codespell -q 3 -I ../opencv-whitelist.txt --skip="./3rdparty"`

Corrected formula for weighted within-class variance

Correct a mistake in {js,py}_table_of_contents_histograms.markdown (#11260)

* Update py_table_of_contents_histograms.markdown

"Learn to find and draw Contours" => "Learn the basics of histograms"

* Update js_table_of_contents_histograms.markdown

Corrected a seemlingly wronged line of copy in the python documention js_table_of_contents_histograms.markdown

"Learn to find and draw Contours" => "Learn the basics of histograms"

Found via `codespell -q 3 --skip="./3rdparty" -I ../opencv-whitelist.txt`

cv2.findContours() no longer modifies source image (#8150)

* cv2.findContours() no longer modifies source image

Since OpenCV 3.2, cv2.findContours() no longer modifies the source image but returns a modified image as the first of three return parameters.  ??? Do I have that correct ???

Also fixed up the code blocks to be valid markdown.

* ```python --> @code{.py}

Enables syntax highlighting in docs.

fix whitespace

mask = cv2.inRange(hsv, lower_green, upper_green)
to
mask = cv2.inRange(hsv, lower_blue, upper_blue)

GSoC Python Tutorials

removed white spaces

removed blank lines at EOF

removed duplicate labels