Set proper text-aligh for the <th> element of the Subtotal column

Author: TomashKhamlai

app/design/frontend/Magento/blank/Magento_Sales/web/css/source/_email.less

@@ -203,7 +203,8 @@
         text-align: center;
     }
 
-    .item-price {
+    .item-price,
+    .item-subtotal {
         text-align: right;
     }
 

app/design/frontend/Magento/luma/Magento_Sales/web/css/source/_email.less

@@ -207,7 +207,8 @@
         text-align: center;
     }
 
-    .item-price {
+    .item-price,
+    .item-subtotal {
         text-align: right;
     }