update plet() documentation

[dfriend21]

The current CRAN version of leafet now supports SpatVector and SpatRaster, so the text in plet stating that the development version is needed is no longer necessary. I made sure this was the case with the following code (I have the CRAN version of leaflet):

library(terra)
v <- vect(system.file("ex/lux.shp", package = "terra"))
r <- rast(system.file("ex/elev.tif", package = "terra"))
plet(v)
plet(svc(v))
plet(r)

[rhijmans]

Thank you!