fix indentations in example

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

README.md

@@ -749,17 +749,17 @@ variable "TAG" {default="" }
 
 group "default" {
 	targets = [
-    "webapp",
+		"webapp",
-  ]
+	]
 }
 
 target "webapp" {
-  context="."
+	context="."
-  dockerfile="Dockerfile"
+	dockerfile="Dockerfile"
-  tags = [
+	tags = [
-      "my-image:latest",
+		"my-image:latest",
-      notequal("",TAG) ? "my-image:${TAG}": "",
+		notequal("",TAG) ? "my-image:${TAG}": "",
-  ]
+	]
 }
 EOF