Fix go modules

Wow. This is fun. Sure is easy to get modules wrong when upgrading a v2+
project.

examples/chat/main.go

@@ -6,7 +6,7 @@ import (
 	"fmt"
 	"os"
 
-	pgxpool "github.com/jackc/pgx/pool"
+	pgxpool "github.com/jackc/pgx/v4/pool"
 )
 
 var pool *pgxpool.Pool

examples/todo/main.go

@@ -6,7 +6,7 @@ import (
 	"os"
 	"strconv"
 
-	"github.com/jackc/pgx"
+	"github.com/jackc/pgx/v4"
 )
 
 var conn *pgx.Conn

examples/url_shortener/main.go

@@ -10,8 +10,8 @@ func main() {
 // 	"net/http"
 // 	"os"
 
-// 	"github.com/jackc/pgx"
-// 	"github.com/jackc/pgx/log/log15adapter"
+// 	"github.com/jackc/pgx/v4"
+// 	"github.com/jackc/pgx/v4/log/log15adapter"
 // 	log "gopkg.in/inconshreveable/log15.v2"
 // )