Fixing bug in flb_plugin.c: key -> value

[QtRoS]

Sorry, there is nothing to fill there, it's obvious bug. Not sure about the fix though, just used exactly the same syntax as in the very next function flb_plugin_load_config_file

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[QtRoS]

@edsiper @leonardo-albertovich @fujimotos @koleini please take a quick look on this one-line fix.

[QtRoS]

@edsiper please take a look