book: shorten tutorial title

doc/src/SUMMARY.md

@@ -25,4 +25,4 @@
 - [Learn](learn/learn.md)
   - [Research](learn/research.md)
   - [ZK explainer](learn/zk_explainer.md)
-  - [Tutorial- Writing a p2p app](learn/writing-a-p2p-app.md)
+  - [Writing a p2p app](learn/writing-a-p2p-app.md)

doc/src/learn/writing-a-p2p-app.md

@@ -1,4 +1,4 @@
-# Tutorial: Writing a p2p chat app
+# Writing a p2p app
 
 This tutorial will teach you how to deploy an app on DarkFi's p2p network.