docs: fixup module structure example

Change-Id: Ic65b2b8fa28a5110b699975b5100228c3f6948aa
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39763
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
diff --git a/docs/module_structure.rst b/docs/module_structure.rst
index aa8cbb7..7334b53 100644
--- a/docs/module_structure.rst
+++ b/docs/module_structure.rst
@@ -304,7 +304,7 @@
   }
 
   config("define_options") {
-    defines = [ "-DPW_FOO_INPUT_BUFFER_SIZE_BYTES=256" ]
+    defines = [ "PW_FOO_INPUT_BUFFER_SIZE_BYTES=256" ]
   }
 
   # This configuration sets PW_FOO_INPUT_BUFFER_SIZE_BYTES in a header file.