Fix pass-by-reference notice

[NateWr]

Clean up another PHP warning. I assume this is all straightforward but can you take a quick look @asmecher?

[asmecher]

No problem with this solution, @NateWr, but in addition (for the next releases) I'd suggest getting rid of the reference requirement in PluginRegistry::register (and probably elsewhere in that class). References are almost never required, especially on objects. (Perhaps the one exception is when when passing around arrays, as we sometimes expect them to be modifiable after they're returned.)

[NateWr]

Filed: pkp/pkp-lib#4061