`grants.added` placeholder is wrong

[NotMyFault]

https://github.com/IntellectualSites/PlotSquared/blob/ff83868cbc20f082362bfcb487b62c865a2926e9/Core/src/main/resources/lang/messages_en.json#L457 is wrong because /plot grant add <player> doesn't accept further arguments and always grants a single plot only. The message should rather be worded like 1 grant has been added, unless the functionality to grant multiple plots is added, which hasn't been the case, ever.