Remove array brackets in super-car example (#177)

Homie 4.0 dropped array support, so super-car example shouldn't include array brackets

Author: stefan-muc

convention.md

@@ -141,7 +141,7 @@ For example, a device with an ID of `super-car` that comprises of a `wheels`, `e
 ```java
 homie/super-car/$homie → "2.1.0"
 homie/super-car/$name → "Super car"
-homie/super-car/$nodes → "wheels,engine,lights[]"
+homie/super-car/$nodes → "wheels,engine,lights"
 homie/super-car/$implementation → "esp8266"
 homie/super-car/$state → "ready"
 ```
@@ -323,4 +323,4 @@ For example, an organization `example.org` wanting to add a feature `our-feature
 
 Every extension must be published using a license.
 The license can be chosen freely, even proprietary licenses are possible.
-The recommended license is the [CCA 4.0](https://homieiot.github.io/license), since this is the license Homie itself uses.
+The recommended license is the [CCA 4.0](https://homieiot.github.io/license), since this is the license Homie itself uses.