fix: button flex style

Author: rayhomie

compiled/alipay/src/Button/index.less

@@ -6,7 +6,7 @@
   position: relative;
   display: flex;
   flex-direction: column;
-  justify-self: center;
+  justify-content: center;
   align-items: center;
   height: auto;
   line-height: normal;

compiled/wechat/src/Button/index.wxss

@@ -2,7 +2,7 @@
   position: relative;
   display: flex;
   flex-direction: column;
-  justify-self: center;
+  justify-content: center;
   align-items: center;
   height: auto;
   line-height: normal;

src/Button/index.less

@@ -6,7 +6,7 @@
   position: relative;
   display: flex;
   flex-direction: column;
-  justify-self: center;
+  justify-content: center;
   align-items: center;
   height: auto;
   line-height: normal;