SUMMARY: replace in-tree/out-of-tree with "mainline"

Using out-of-tree may sound like we are referring to modules that are
actually developed out-of-tree rather than in-tree in a fork.

Thus be precise and just mention mainline which is anyway clearer.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

SUMMARY.md

@@ -29,14 +29,14 @@
   - [`rustc_codegen_gcc`](rustc_codegen_gcc.md)
   - [`gccrs`](gccrs.md)
 
-## Users — in-tree
+## Users — in mainline
 
   - [AMCC QT2025 PHY Driver](AMCC-QT2025-PHY-Driver.md)
   - [ASIX PHY Driver](ASIX-PHY-Driver.md)
   - [DRM Panic QR code generator](DRM-Panic-QR-code-generator.md)
   - [Null Block Driver](Null-Block-Driver.md)
 
-## Users — out-of-tree
+## Users — outside mainline
 
   - [Android `ashmem`](Android-`ashmem`.md)
   - [Android Binder Driver](Android-Binder-Driver.md)