[http] fix documentation for channel modes (#9593)

Based on https://github.com/openhab/openhab-addons/blob/8de5652ed19775900ae1ee4c9e59686590112db9/bundles/org.openhab.binding.http/src/main/java/org/openhab/binding/http/internal/config/HttpChannelMode.java#L23 and some experimenting, the correct values are WRITEONLY and READONLY

bundles/org.openhab.binding.http/README.md

@@ -42,7 +42,7 @@ The `image` channel-type supports `stateExtension` only.
 | `commandExtension`      | yes      |      -      | Appended to the `baseURL` for sending commands. If empty, same as `stateExtension`. |
 | `stateTransformation  ` | yes      |      -      | One or more transformation applied to received values before updating channel. |
 | `commandTransformation` | yes      |      -      | One or more transformation applied to channel value before sending to a remote. |
-| `mode`                  | no       | `READWRITE` | Mode this channel is allowed to operate. `READ` means receive state, `WRITE` means send commands. |
+| `mode`                  | no       | `READWRITE` | Mode this channel is allowed to operate. `READONLY` means receive state, `WRITEONLY` means send commands. |
 
 Transformations need to be specified in the same format as 
 Some channels have additional parameters.
@@ -147,4 +147,4 @@ is transformed to
 
 ```
 http://www.domain.org/home/lights/23871/?status=OFF&date=2020-07-06
-```
+```