Fix three docstring inconsistencies per PR review

Three CoPilot findings on PR #116:

1. main.py docstring referenced PlaceholderSegment(name="history")
   but the actual field is named ``placeholder``. The code's
   chat-template construction uses ``placeholder=`` correctly; only
   the docstring narrative was wrong. A reader copy-pasting from the
   docstring would have hit a Pydantic ValidationError.
2. Walk-through doc's intro paragraph still said "Apollo 11 Lunar
   Module" after the image URL swap to the Apollo 16 "Orion" shot.
   The code's docstring + inline comment got updated when the URL
   changed but the walk-through intro was missed.
3. Sample "Reading the output" block showed an ``upload.wikimedia.org``
   image URL the example explicitly warns against. Updated to the
   actual default ``images-assets.nasa.gov`` URL so the sample
   matches a real run.

Author: chris-colinsky

docs/examples/11-chat-with-multimodal.md

@@ -2,9 +2,9 @@
 
 A lunar-mission Q&A assistant that maintains conversation context
 across four turns. One mid-conversation turn includes an attached
-photograph (Apollo 11 Lunar Module on the lunar surface): the user
-asks about it, the agent processes the multimodal turn naturally
-without changing the chat-history shape.
+photograph (Apollo 16 Lunar Module "Orion" on the lunar surface):
+the user asks about it, the agent processes the multimodal turn
+naturally without changing the chat-history shape.
 
 ## Overview
 
@@ -115,7 +115,7 @@ LLM, and updates state.
 
 ```
 === openarmature chat-with-multimodal demo ===
-Image URL: https://upload.wikimedia.org/...
+Image URL: https://images-assets.nasa.gov/image/as16-113-18334/...
 Scripted turns: 4
 
 --- Turn 0 ---

examples/11-chat-with-multimodal/main.py

@@ -24,7 +24,7 @@
   which switches the user `ContentSegment.content` between a single
   text template (text-only turn) and a `[TextBlockTemplate,
   ImageURLBlockTemplate]` list (multimodal turn). The system segment
-  and the `PlaceholderSegment(name="history")` slot are identical
+  and the `PlaceholderSegment(placeholder="history")` slot are identical
   across both shapes; only the trailing user segment changes.
 - Chat history lives on state as `history: Annotated[list[Message],
   append]`. After each turn the node appends two messages (the new