Fix indention level for --no-python-gen-numpy (#9049)

src/idl_gen_python.cpp

diff --git a/src/idl_gen_python.cpp b/src/idl_gen_python.cpp
index fa37870..cb25fbc 100644
--- a/src/idl_gen_python.cpp
+++ b/src/idl_gen_python.cpp
@@ -2471,7 +2471,7 @@
     } else {
       GenPackForScalarVectorFieldHelper(struct_def, field, code_prefix_ptr, 3);
       code_prefix += "(self." + field_field + "[i])";
-      code_prefix += GenIndents(4) + field_field + " = builder.EndVector()";
+      code_prefix += GenIndents(3) + field_field + " = builder.EndVector()";
     }
   }