azurerm_express_route_circuit_peering: fix testAccAzureRMExpressRouteCircuitPeering_azurePrivatePeering acc test

[katbyte]

before:

 --- FAIL: TestAccAzureRMExpressRouteCircuit/PrivatePeering/azurePrivatePeering (149.07s)
        	testing.go:538: Step 1 error: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.
        		
        		(map[string]string) {
        		}
        		
        		
        		(map[string]string) (len=1) {
        		 (string) (len=10) "shared_key": (string) (len=24) "ABCdefGHIJklm@nOPqrsTU!!"
        		}
        		

after

--- PASS: TestAccAzureRMExpressRouteCircuit (160.52s)
    --- PASS: TestAccAzureRMExpressRouteCircuit/PrivatePeering (0.00s)
        --- PASS: TestAccAzureRMExpressRouteCircuit/PrivatePeering/azurePrivatePeering (160.52s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	162.158s

[ghost]

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