docs: fix typo in image adjustment comment (Brighntess -> Brightness)

- Fix spelling error in DisplayRenderer.cc comment section
- Change "Brighntess" to "Brightness" in image adjustment logic comment

Author: JWWTSL

qt/src/DisplayRenderer.cc

@@ -54,7 +54,7 @@ void DisplayRenderer::adjustImage(QImage& image, int brightness, int contrast, b
 			int red = qRed(rgb[i]);
 			int green = qGreen(rgb[i]);
 			int blue = qBlue(rgb[i]);
-			// Brighntess
+			// Brightness
 			red = dBr * (1.0 - kBr) + red * kBr;
 			green = dBr * (1.0 - kBr) + green * kBr;
 			blue = dBr * (1.0 - kBr) + blue * kBr;