Fix allowed values in StringArgument mentioned in the documentation (CommandAPI#581)

Author: MC-XiaoHei

docssrc/src/argument_strings.md

@@ -6,14 +6,16 @@ There are three types of arguments that return Java's `String` object. Each have
 
 ## String argument
 
-The `StringArgument` class is used to represent a single word. These words **can only contain alphanumeric characters (A-Z, a-z and 0-9), and the underscore character**.
+The `StringArgument` class is used to represent a single word. These words **can only contain alphanumeric characters (A-Z, a-z and 0-9) and the underscore (_), plus (+), minus (-) and period (.) characters.**
 
 Accepted `StringArgument` values:
 
 ```txt
 Hello
 123
 hello123
+hello-123
+hello.WORLD
 Hello_world
 ```