Fix document issue for test and example related to resource principal.

Author: pengfei0107

examples/parameters.py

@@ -127,7 +127,7 @@
 password = None
 
 # Cloud Simulator/on-premise: not used
-# Service: 'user principal', 'instance principal' or 'resource principals'
+# Service: 'user principal', 'instance principal' or 'resource principal'
 principal = None
 
 # Cloud Simulator/on-premise: not used

test/parameters.py

@@ -78,7 +78,7 @@
 
 
 def iam_principal():
-    # Use 'user principal', 'instance principal' or 'resource principals' for
+    # Use 'user principal', 'instance principal' or 'resource principal' for
     # production pod, and use None for minicloud testing.
     return None