Add a single-line comment example

spec/abstract-semantic-graph/examples/comment-single-line.adoc

+Let me show you the way.
+
+// A single-line comment.
+
+Let's go on a little trip.
\ No newline at end of file

spec/abstract-semantic-graph/examples/comment-single-line.asg.json

+{
+  "body": [
+    {
+      "type": "Paragraph",
+      "children": [
+        {
+          "type": "Str",
+          "value": "Let me show you the way."
+        }
+      ]
+    },
+    {
+      "type": "Paragraph",
+      "children": [
+        {
+          "type": "Str",
+          "value": "Let's go on a little trip."
+        }
+      ]
+    }
+  ]
+}