[cli] fix network time format specifier (#3278)

diff --git a/src/cli/cli.cpp b/src/cli/cli.cpp
index a88bb3b..a46a198 100644
--- a/src/cli/cli.cpp
+++ b/src/cli/cli.cpp
@@ -1624,7 +1624,7 @@
 
         networkTimeStatus = otNetworkTimeGet(mInstance, time);
 
-        mServer->OutputFormat("Network Time:     %dus", time);
+        mServer->OutputFormat("Network Time:     %luus", time);
 
         switch (networkTimeStatus)
         {