Clarify system identity is a flag

Author: wchigit

src/azure-cli/azure/cli/command_modules/serviceconnector/_help.py

@@ -320,7 +320,7 @@ def get_source_display_name(sourcename):
             else:
                 system_identity_param = '''
             - name: --system-identity
-              short-summary: The system assigned identity auth info
+              short-summary: The flag to use system assigned identity auth info. 
               long-summary: |
                 Usage: --system-identity