Fixed Misleading main class error message

if your plugin filename (without ext) is not the same as the main :RustPlugin class name the server returns error such as this:

Code:

[Oxide] 1:00 AM [Info] Loaded plugin Player location v1.2.0 by #Domestos
[Oxide] 1:00 AM [Error] Plugin filename is incorrect: Toolk-005bp.cs
[Oxide] 1:00 AM [Error] Plugin filename is incorrect: Toolk-008bp.cs

this my potentially lead to people frantically trying to find any potential illegal characters in their filenames. this is not fun when the plugin has to be uploaded to a slow ftp server each time, don't ask me how i know.

Please consider putting an information that the class name and the file name have to be the same, in the error message and in the documentation.