chore(theme-default): remove deprecated ~ mark in stylus

Author: meteorlxy

packages/@vuepress/theme-default/src/styles/_variables.styl

@@ -1,5 +1,5 @@
 // palette-stylus
-@require '~@temp/palette';
+@require '@temp/palette';
 
 // colors
 $accentColor ?= #3eaf7c;

packages/@vuepress/theme-default/src/styles/index.styl

@@ -18,4 +18,4 @@
 @require './transitions';
 
 // palette-stylus
-@require '~@temp/style';
+@require '@temp/style';