docs: Explicit temporal type (vega#8352)

Explicit temporal type

Since this spec is used as the example for the [temporal encoding type](https://vega.github.io/vega-lite/docs/type.html#temporal) it's nice to have it spelled out.

Co-authored-by: Dan Marshall <danmar@microsoft.com>

examples/specs/bar_month_temporal.vl.json

@@ -3,7 +3,7 @@
   "data": {"url": "data/seattle-weather.csv"},
   "mark": "bar",
   "encoding": {
-    "x": {"timeUnit": "month", "field": "date"},
+    "x": {"timeUnit": "month", "field": "date", "type": "temporal"},
     "y": {"aggregate": "mean", "field": "precipitation"}
   }
 }