Dateformat epoch_mills is mispelled

Following nest call fails:
elasticClient.CreateIndex("testindex",
                        descriptor =>
                            descriptor.Mappings(mappings => mappings
                                .Map<EventMapping>(md => md
                                    .Properties(props => props
                                        .Date(d => d.Name("Time").Format(DateFormat.epoch_millis))
...

```
{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "Failed to parse mapping [event]: Invalid format: [epoch_mills]: Illegal pattern component: p"
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "Failed to parse mapping [event]: Invalid format: [epoch_mills]: Illegal pattern component: p",
    "caused_by": {
      "type": "illegal_argument_exception",
      "reason": "Invalid format: [epoch_mills]: Illegal pattern component: p",
      "caused_by": {
        "type": "illegal_argument_exception",
        "reason": "Illegal pattern component: p"
      }
    }
  },
  "status": 400
}
```

Author: JefStat

src/Nest/CommonOptions/DateFormat/DateFormat.cs

@@ -5,7 +5,7 @@
 	public static class DateFormat
 	{
 		///<summary>A formatter for the number of milliseconds since the epoch. Note, that this timestamp is subject to the limits of a Java Long.MIN_VALUE and Long.MAX_VALUE.</summary>
-		public const string epoch_millis = "epoch_mills";
+		public const string epoch_millis = "epoch_millis";
 		///<summary>A formatter for the number of seconds since the epoch. Note, that this timestamp is subject to the limits of a Java Long.MIN_VALUE and Long. MAX_VALUE divided by 1000 (the number of milliseconds in a second).</summary>
 		public const string epoch_second = "epoch_second";
 		///<summary>A basic formatter for a full date as four digit year, two digit month of year, and two digit day of month: yyyyMMdd.</summary>