Change `add-ons` to `addons` for consistency

docs/guides/getting_started/installing.md

@@ -27,7 +27,7 @@ required. When using .NET Framework, it is suggested to target
 Release builds of Discord.Net 1.0 will be published to the
 [official NuGet feed].
 
-Development builds of Discord.Net 1.0, as well as add-ons *(TODO)* are
+Development builds of Discord.Net 1.0, as well as addons *(TODO)* are
 published to our development [MyGet feed].
 
 Direct feed link: `https://www.myget.org/F/discord-net/api/v3/index.json`

docs/guides/getting_started/terminology.md

@@ -23,8 +23,8 @@ but you should be aware of the differences between them.
 `Discord.Net.Core` provides a set of interfaces that models Discord's 
 API. These interfaces are consistent throughout all implementations of 
 Discord.Net, and if you are writing an implementation-agnostic library 
-or add-on, you can rely on the core interfaces to ensure that your 
-add-on will run on all platforms.
+or addon, you can rely on the core interfaces to ensure that your 
+addon will run on all platforms.
 
 `Discord.Net.Rest` provides a set of concrete classes to be used 
 **strictly** with the REST portion of Discord's API. Entities in this