[EC2] Cosmetic fix for usage of spark-ec2 --ebs-vol-num option

The last line of the usage seems ugly.

```
$ spark-ec2 --help
<snip>
  --ebs-vol-num=EBS_VOL_NUM
                        Number of EBS volumes to attach to each node as
                        /vol[x]. The volumes will be deleted when the
                        instances terminate. Only possible on EBS-backed AMIs.
                        EBS volumes are only attached if --ebs-vol-size >
                        0.Only support up to 8 EBS volumes.
```

After applying this patch:

```
$ spark-ec2 --help
<snip>
  --ebs-vol-num=EBS_VOL_NUM
                        Number of EBS volumes to attach to each node as
                        /vol[x]. The volumes will be deleted when the
                        instances terminate. Only possible on EBS-backed AMIs.
                        EBS volumes are only attached if --ebs-vol-size > 0.
                        Only support up to 8 EBS volumes.
```

As this is a trivial thing I didn't create JIRA for this.

Author: Kenichi Maehashi <webmaster@kenichimaehashi.com>

Closes #7632 from kmaehashi/spark-ec2-cosmetic-fix and squashes the following commits:

526c118 [Kenichi Maehashi] cosmetic fix for spark-ec2 --ebs-vol-num option usage

ec2/spark_ec2.py

@@ -242,7 +242,7 @@ def parse_args():
         help="Number of EBS volumes to attach to each node as /vol[x]. " +
              "The volumes will be deleted when the instances terminate. " +
              "Only possible on EBS-backed AMIs. " +
-             "EBS volumes are only attached if --ebs-vol-size > 0." +
+             "EBS volumes are only attached if --ebs-vol-size > 0. " +
              "Only support up to 8 EBS volumes.")
     parser.add_option(
         "--placement-group", type="string", default=None,