fix readme for maxrange (#94)

READSB_MAX_RANGE has two entries in the readme, remove one of them.

The default value is not a container but rather a readsb default value.
It's 450 nmi, not 300 nmi.
Correct that fact.

README.md

@@ -196,11 +196,10 @@ The following parameters must be set (mandatory) for the container to function:
 | `READSB_EXTRA_ARGS`           | Optional, allows to specify extra parameters for readsb                                                                          | Unset                          |           |
 | `READSB_DEBUG`                | Optional, used to set debug mode. `n`: network, `P`: CPR, `S`: speed check                                                       | Unset                          |           |
 | `S6_SERVICES_GRACETIME`       | Optional, set to 30000 when saving traces / globe_history                                                                        | `3000`                         |           |
-| `READSB_MAX_RANGE`            | Optional. Maximum range (in nautical miles).                                                                                     | `300`                          |           |
 | `READSB_ENABLE_BIASTEE`       | Set to `true` to enable bias tee on supporting interfaces                                                                        |                                | Unset     |
 | `READSB_RX_LOCATION_ACCURACY` | Accuracy of receiver location in metadata: 0=no location, 1=approximate, 2=exact                                                 | `--rx-location-accuracy=<n>`   | `2`       |
 | `READSB_HEATMAP_INTERVAL`     | Per plane interval for heatmap and replay (if you want to lower this, also lower json-trace-interval to this or a lower value)   | `--heatmap=<sec>`              | `15`      |
-| `READSB_MAX_RANGE`            | Absolute maximum range for position decoding (in nm)                                                                             | `--max-range=<dist>`           | `300`     |
+| `READSB_MAX_RANGE`            | Absolute maximum range for position decoding (in nm)                                                                             | `--max-range=<dist>`           | `450`     |
 | `READSB_STATS_EVERY`          | Number of seconds between showing and resetting stats.                                                                           | `--stats-every=<sec>`          | Unset     |
 | `READSB_STATS_RANGE`          | Set this to `true` to collect range statistics for polar plot.                                                                   | `--stats-range`                | Unset     |
 | `READSB_RANGE_OUTLINE_HOURS`  | Change which past timeframe the range outline is based on                                                                        | `--range-outline-hours`        | `24`      |