Fix AllowableValues failProp error

The RedfishServiceValidator throws the following error:
"ERROR - Locked@Redfish.AllowableValues[0]: Expected
string value, got type class 'bool'"

From http://redfish.dmtf.org/schemas/DSP0266_1.0.html#allowable-values

"6.5.4.7.2. Allowable values

The property representing the action may be annotated with the
"AllowableValues" annotation in order to specify the list of
allowable values for a particular parameter.

The set of allowable values is specified by including a property
whose name is the name of the parameter followed by
"@Redfish.AllowableValues", and whose value is a JSON array of
strings representing the allowable values for the parameter."

Moved AllowableValues[0] to a string.

Tested: Ran RedfishServiceValidator and no longer observe the
        error.

Change-Id: I167416fe62449dd428f549c486069db0de7676b0
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/redfish-core/lib/account_service.hpp b/redfish-core/lib/account_service.hpp
index a5c501d..70eed78 100644
--- a/redfish-core/lib/account_service.hpp
+++ b/redfish-core/lib/account_service.hpp
@@ -477,7 +477,7 @@
                                     *userLocked;
                                 asyncResp->res.jsonValue
                                     ["Locked@Redfish.AllowableValues"] = {
-                                    false};
+                                    "false"};
                             }
                             else if (property.first == "UserPrivilege")
                             {