json: format output with 4-space indentation

Use `dump(4)` instead of `dump()` to generate JSON with
4-space indentation, improving readability of written files.

Change-Id: I7aad3bf50cdf482dde7c0bdd5aa76d030aa0ca60
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/src/password.cpp b/src/password.cpp
index abde4b5..97b44bf 100644
--- a/src/password.cpp
+++ b/src/password.cpp
@@ -223,7 +223,7 @@
         json["IsAdminPwdChanged"] = true;
 
         std::ofstream ofs(seedFile.c_str(), std::ios::out);
-        const auto& writeData = json.dump();
+        const auto& writeData = json.dump(4);
         ofs << writeData;
         ofs.close();
     }