fix issue 9359

doc/tutorials/videoio/video-input-psnr-ssim/video_input_psnr_ssim.markdown

@@ -133,7 +133,7 @@ Then the PSNR is expressed as:
 
 \f[PSNR = 10 \cdot \log_{10} \left( \frac{MAX_I^2}{MSE} \right)\f]
 
-Here the \f$MAX_I^2\f$ is the maximum valid value for a pixel. In case of the simple single byte image
+Here the \f$MAX_I\f$ is the maximum valid value for a pixel. In case of the simple single byte image
 per pixel per channel this is 255. When two images are the same the MSE will give zero, resulting in
 an invalid divide by zero operation in the PSNR formula. In this case the PSNR is undefined and as
 we'll need to handle this case separately. The transition to a logarithmic scale is made because the