alacritty use window.opacity instead of background_opacity

dotfiles/alacritty/.config/alacritty/alacritty.yml

@@ -12,7 +12,13 @@
   # available, otherwise `xterm-256color` is used.
   #TERM: xterm-256color
 
-#window:
+window:
+  # Background opacity
+  #
+  # Window opacity as a floating point number from `0.0` to `1.0`.
+  # The value `0.0` is completely transparent and `1.0` is opaque.
+  opacity: 0.9
+
   # Window dimensions (changes require restart)
   #
   # Specified in number of columns/lines, not pixels.
@@ -286,12 +292,6 @@ font:
 #  duration: 0
 #  color: '0xffffff'
 
-# Background opacity
-#
-# Window opacity as a floating point number from `0.0` to `1.0`.
-# The value `0.0` is completely transparent and `1.0` is opaque.
-background_opacity: 0.9
-
 #selection:
   #semantic_escape_chars: ",│`|:\"' ()[]{}<>\t"