doc: samples: bme280: fix missing end of sentence

Looks like I forgot to finish a sentence. There's nothing really
special besides the top level requirement for an enabled node which is
mentioned at the top of the page. Add a reference to the bindings
index while we're here to make it clearer what such nodes contain.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>