doc: adapt image generation README to inkscape / imagemagick changes.

Heh, interesting how drastically random software changes over the years.

doc/generated/README.md

@@ -24,14 +24,14 @@ is put into `doc/` directory. Apply `pngcrush` to them for smaller file sizes:
 Generated by the `shaders` executable. Must be run in this directory, the
 output is put into `doc/` directory. The executable requires two textures:
 
--   `vector.png`, generated as full-page PNG output at 90 DPI from `vector.svg`,
+-   `vector.png`, generated as full-page PNG output at 96 DPI from `vector.svg`,
     converted to pure grayscale using imagemagick:
 
     ```bash
-    mogrify -flatten -background '#ffffff' -format grayscale vector.png
+    mogrify -flatten -background '#ffffff' -colorspace gray vector.png
     ```
 
--   `vector-distancefield.png`, generated as full-page PNG output at 360 DPI
+-   `vector-distancefield.png`, generated as full-page PNG output at 384 DPI
     (1024x1024) to `vector-src.png` and then processed through
     `magnum-distancefieldconverter`