fix groupBy docs

docs/graphql-api.md

···
 ```graphql
 query TopTracks {
   fmTealAlphaFeedPlaysAggregated(
-    groupBy: [trackName]
+    groupBy: [{ field: trackName }]
     orderBy: { count: desc }
     limit: 10
   ) {
···
 ```graphql
 query TopTracksByArtist {
   fmTealAlphaFeedPlaysAggregated(
-    groupBy: [trackName, artists]
+    groupBy: [{ field: trackName }, { field: artists }]
     orderBy: { count: desc }
     limit: 20
   ) {
···
 ```graphql
 query TopTracksThisWeek {
   fmTealAlphaFeedPlaysAggregated(
-    groupBy: [trackName, artists]
+    groupBy: [{ field: trackName }, { field: artists }]
     where: {
       indexedAt: {
         gte: "2025-01-01T00:00:00Z"
···
 query DailyPlays {
   fmTealAlphaFeedPlaysAggregated(
     groupBy: [
-      { field: "playedTime", interval: day }
+      { field: playedTime, interval: day }
     ]
     orderBy: { count: desc }
     limit: 30
···
 query TrackPlaysByDay {
   fmTealAlphaFeedPlaysAggregated(
     groupBy: [
-      { field: "trackName" },
-      { field: "playedTime", interval: day }
+      { field: trackName },
+      { field: playedTime, interval: day }
     ]
     orderBy: { count: desc }
     limit: 100
···
 ```graphql
 query WeeklyPlays {
   fmTealAlphaFeedPlaysAggregated(
-    groupBy: [trackName]
+    groupBy: [{ field: trackName }]
     where: {
       playedTime: {
         gte: "2025-01-01T00:00:00Z"
···
 ```graphql
 query TrendingArtists {
   fmTealAlphaFeedPlaysAggregated(
-    groupBy: [artists]
+    groupBy: [{ field: artists }]
     where: {
       playedTime: { gte: "2025-01-01T00:00:00Z" }
     }