Fix mermaid syntax in tutorial 03 diagram

`--|label|` is not valid mermaid; the labelled-arrow form is
`-->|label|` with the arrow head present.

Author: darmie

docs/tutorials/real-world/03-multi-agent.md

@@ -13,14 +13,14 @@ flowchart LR
     topic --> factual[factual_researcher]
     topic --> stat[statistician]
     topic --> quote[quoter]
-    factual --|chunk| logger[logger]
-    stat    --|chunk| logger
-    quote   --|chunk| logger
-    factual --|finding| synth[synthesizer]
-    stat    --|finding| synth
-    quote   --|finding| synth
-    synth   --|chunk|  logger
-    synth   --|answer| sink[sink]
+    factual -->|chunk| logger[logger]
+    stat    -->|chunk| logger
+    quote   -->|chunk| logger
+    factual -->|finding| synth[synthesizer]
+    stat    -->|finding| synth
+    quote   -->|finding| synth
+    synth   -->|chunk|  logger
+    synth   -->|answer| sink[sink]
 ```
 
 Two properties of the graph matter. The three specialists run