Update postcss.rst

Use the browserlists default browsers as reccommended by "browserlists"

```defaults`` includes the already mentioned settings on this page and is the reccommended value for most users, listing all the including values as a reccommendation from Symfony is not helpful. I included the equivilent of defaults below the new reccommendation for explaination.

Author: Taminoful

frontend/encore/postcss.rst

@@ -63,6 +63,16 @@ The ``autoprefixer`` (and many other tools) need to know what browsers you want
 support. The best-practice is to configure this directly in your ``package.json``
 (so that all the tools can read this):
 
+.. code-block:: diff
+
+    {
+    +  "browserslist": [
+    +    "defaults",
+    +  ]
+    }
+
+The ``defaults`` option is reccommended for most users and would be equivilant to the following browserslist.
+
 .. code-block:: diff
 
     {