LibWeb: Make the linter happy in Properties.json

There was a space missing and the linter complained.

Userland/Libraries/LibWeb/CSS/Properties.json

@@ -140,7 +140,7 @@
     "valid-types": [
       "ratio"
     ],
-    "valid-identifiers":[
+    "valid-identifiers": [
       "auto"
     ]
   },