docs: correct the sizes the 4 KiB rebuild produced

The databases came out of the first 4 KiB build at 288.6 MB and 237.7
MB, against the 302 and 247 the registry carried. That figure is a build
guard and is also shown to the user before the SQL tab opens, so both
uses were wrong by the same 5%.

The gzip variant the host builds for the HEAD request is now 64 MB, and
the published size claims in the deployment and architecture notes
follow the same rebuild.

datasets.json

@@ -20,12 +20,12 @@
     {
       "id": "2016",
       "expectedRows": 877460,
-      "dbSizeMb": 302
+      "dbSizeMb": 288
     },
     {
       "id": "2017",
       "expectedRows": 861068,
-      "dbSizeMb": 247
+      "dbSizeMb": 237
     }
   ]
 }