Replace relative_target_gen_dir with target_gen_dir in docs

It seems relative_target_gen_dir was removed some time ago:
https://codereview.chromium.org/23606031

Change-Id: I8f58291607945a54bbe2cd21b533d0868ec6f974
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/10880
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

docs/reference.md

diff --git a/docs/reference.md b/docs/reference.md
index 4cced58..80151ba 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -1464,7 +1464,7 @@
     args = [
       "{{source}}",
       "-o",
-      rebase_path(relative_target_gen_dir, root_build_dir) +
+      rebase_path(target_gen_dir, root_build_dir) +
         "/{{source_name_part}}.h" ]
   }
 ```

src/gn/functions_target.cc

diff --git a/src/gn/functions_target.cc b/src/gn/functions_target.cc
index f74cfa6..7693369 100644
--- a/src/gn/functions_target.cc
+++ b/src/gn/functions_target.cc
@@ -256,7 +256,7 @@
     args = [
       "{{source}}",
       "-o",
-      rebase_path(relative_target_gen_dir, root_build_dir) +
+      rebase_path(target_gen_dir, root_build_dir) +
         "/{{source_name_part}}.h" ]
   }
 )";