Clarify market-type option in README (machulav#250)

Author: GabLeRoux

README.md

@@ -216,7 +216,7 @@ Now you're ready to go!
 | `aws-resource-tags`                                                                                                                                                          | Optional. Used only with the `start` mode. | Specifies tags to add to the EC2 instance and any attached storage. <br><br> This field is a stringified JSON array of tag objects, each containing a `Key` and `Value` field (see example below). <br><br> Setting this requires additional AWS permissions for the role launching the instance (see above).                         |
 | `runner-home-dir`                                                                                                                                                              | Optional. Used only with the `start` mode. | Specifies a directory where pre-installed actions-runner software and scripts are located.<br><br> |
 | `pre-runner-script`                                                                                                                                                              | Optional. Used only with the `start` mode. | Specifies bash commands to run before the runner starts.  It's useful for installing dependencies with apt-get, yum, dnf, etc. For example:<pre>          - name: Start EC2 runner<br>            with:<br>              mode: start<br>              ...<br>              pre-runner-script: \|<br>                 sudo yum update -y && \ <br>                 sudo yum install docker git libicu -y<br>                 sudo systemctl enable docker</pre> |
-| `market-type` | Optional. Used only with the `start` mode. | The only valid option is `spot`. If `spot` is specified, a Spot instance will be requested. If left unspecified, an on-demand instance will be provisioned. |
+| `market-type` | Optional. Used only with the `start` mode. | This field accepts only the value `spot`. <br><br> If set to `spot`, the runner will be launched as a Spot instance. <br><br> If omitted, the runner will be launched as an on-demand instance. |
 | `block-device-mappings` | Optional. Used only with the `start` mode. | JSON string specifying the block device mappings for the EC2 instance. For example: <br> <pre>[{"DeviceName": "/dev/sda1", "Ebs": {"VolumeSize": 100, "VolumeType": "gp3"}}]</pre> See <a href="https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_BlockDeviceMapping.html">AWS BlockDeviceMapping docs</a> for all options. |
 | `startup-quiet-period-seconds` | Optional | Default: 30 |
 | `startup-retry-interval-seconds` | Optional | Default: 10 |