Fix section dividers. (#2750)

They should be consistently between days, but some were between titles
and contents.

Author: qwandor

src/SUMMARY.md

@@ -229,10 +229,10 @@
   - [Exercise: FFI Wrapper](unsafe-rust/exercise.md)
     - [Solution](unsafe-rust/solution.md)
 
-# Android
-
 ---
 
+# Android
+
 - [Welcome](android.md)
 - [Setup](android/setup.md)
 - [Build Rules](android/build-rules.md)
@@ -280,10 +280,10 @@
     - [Building for Android: Rust](android/interoperability/cpp/android-build-rust.md)
   - [With Java](android/interoperability/java.md)
 
-# Chromium
-
 ---
 
+# Chromium
+
 - [Welcome](chromium.md)
 - [Setup](chromium/setup.md)
 - [Comparing Chromium and Cargo Ecosystems](chromium/cargo.md)
@@ -322,10 +322,10 @@
 - [Bringing It Together - Exercise](exercises/chromium/bringing-it-together.md)
 - [Exercise Solutions](exercises/chromium/solutions.md)
 
-# Bare Metal: Morning
-
 ---
 
+# Bare Metal: Morning
+
 - [Welcome](bare-metal.md)
 - [`no_std`](bare-metal/no_std.md)
   - [A Minimal Example](bare-metal/minimal.md)
@@ -375,10 +375,10 @@
   - [RTC Driver](exercises/bare-metal/rtc.md)
   - [Solutions](exercises/bare-metal/solutions-afternoon.md)
 
-# Concurrency: Morning
-
 ---
 
+# Concurrency: Morning
+
 - [Welcome](concurrency/welcome.md)
 - [Threads](concurrency/threads.md)
   - [Plain Threads](concurrency/threads/plain.md)
@@ -424,10 +424,10 @@
   - [Broadcast Chat Application](concurrency/async-exercises/chat-app.md)
   - [Solutions](concurrency/async-exercises/solutions.md)
 
-# Final Words
-
 ---
 
+# Final Words
+
 - [Thanks!](thanks.md)
 - [Glossary](glossary.md)
 - [Other Resources](other-resources.md)