Docs: Fix windows setup instructions

Fix windows instructions for setting pw_target_config build argument.

Change-Id: I96657f3897419ae9446ff067930ba43e83e66c00
diff --git a/docs/getting_started.md b/docs/getting_started.md
index 625c680..18f2e35 100644
--- a/docs/getting_started.md
+++ b/docs/getting_started.md
@@ -245,7 +245,7 @@
 
 **Windows**
 ```batch
-> gn gen "pw_target_config = ""//targets/stm32f429i-disc1/target_config.gni""" out/disco
+> gn gen --args="pw_target_config = ""//targets/stm32f429i-disc1/target_config.gni""" out/disco
 ```
 
 Switch to the window running `pw_watch`, and quit using `ctrl+c`.