writing_gn_templates.md: s/leaves/roots/

Follow-up from original review.

Change-Id: I375b2f266d3712f20efc769fc822df67701297a3
Reviewed-on: https://chromium-review.googlesource.com/1246836
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594462}

Author: agrieve

build/docs/writing_gn_templates.md

@@ -61,7 +61,7 @@ gn analyze //out/Debug <(echo '{
 Do not list files as `outputs` unless they are important. Outputs are important
 if they are:
   * used as an input by another target, or
-  * are leaves in the dependency graph (e.g. binaries, apks, etc).
+  * are roots in the dependency graph (e.g. binaries, apks, etc).
 
 Example:
 * An action runs a binary that creates an output as well as a log file. Do not