Fix README code example error
Fix README code example error

A missing semicolon made the example code in the README fail when trying
to compile.

Signed-off-by: Nick Stevens <75ef9faee755c70589550b513ad881e5a603182c@bitcurry.com>

jwilm/rust-sysfs-gpio,nastevens/rust-sysfs-gpio,rust-embedded/rust-sysfs-gpio,marjakm/rust-sysfs-gpio