Update webrtc build command instruction to autoninja.

`autoninja` is a wrapper that automatically provides optimal values
for the arguments passed to `ninja`. For example, more cores can be
deployed to make webrtc build faster.

Bug: None
Change-Id: I1c62141c10c176e30bbb75994a7a241c0938e6bb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/285044
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38732}
diff --git a/docs/native-code/development/index.md b/docs/native-code/development/index.md
index f3cfd55..f8c65b2 100644
--- a/docs/native-code/development/index.md
+++ b/docs/native-code/development/index.md
@@ -116,15 +116,17 @@
 For [Ninja][ninja] project files generated in `out/Default`:
 
 ```
-$ ninja -C out/Default
+$ autoninja -C out/Default
 ```
 
 To build everything in the generated folder (`out/Default`):
 
 ```
-$ ninja all -C out/Default
+$ autoninja all -C out/Default
 ```
 
+`autoninja` is a wrapper that automatically provides optimal values for the arguments passed to `ninja`.
+
 See [Ninja build rules][ninja-build-rules] to read more about difference between `ninja` and `ninja all`.