etc/mpv.conf: replace the --sharpen example

gpu-next doesn't support --sharpen and it is now the default VO, so
replace it with something that actually works.

etc/mpv.conf

@@ -140,4 +140,4 @@
 # The following profile can be enabled on the command line with: --profile=eye-cancer
 
 #[eye-cancer]
-#sharpen=5
+#gamma=100