Some examples incorrectly use the Image color rather than background color

[alice-i-cecile]

          I guess it was missed

bevy/examples/ui/button.rs

Line 32 in 336fddb

image.color = PRESSED_BUTTON;

bevy/examples/ui/button.rs

Line 37 in 336fddb

image.color = HOVERED_BUTTON;

bevy/examples/ui/button.rs

Line 42 in 336fddb

image.color = NORMAL_BUTTON;

Originally posted by @lufog in #14017 (review)