Update box-sizing.md

docs/uni-app-x/css/box-sizing.md

@@ -2,6 +2,8 @@
 
 
 <!-- CSSJSON.box-sizing.description -->
+### App平台差异  
+App平台默认值为border-box
 
 <!-- CSSJSON.box-sizing.syntax -->
 
@@ -9,4 +11,4 @@
 
 <!-- CSSJSON.box-sizing.compatibility -->
 
-<!-- CSSJSON.box-sizing.reference -->
\ No newline at end of file
+<!-- CSSJSON.box-sizing.reference -->