staging: pi433: Fix typo in documentation

Fixes a typo in the documentation.

Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
Link: https://lore.kernel.org/r/20190905132219.GA1004@SARKAR


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>