Fix blink example

Author: rahul-thakoor

README.md

@@ -22,8 +22,12 @@ use rust_gpiozero::*;
 fn main() {
     // Create a new LED attached to Pin 17
     let mut led = LED::new(17);
-    // blink the LED
-    led.blink(2.0, 3.0);
+
+    // on_time = 2 secs, off_time=3 secs
+    led.blink(2.0,3.0);
+
+    // prevent program from exiting immediately
+    led.wait();
 }
 
 ```

examples/blink.rs

@@ -5,6 +5,10 @@ use rust_gpiozero::*;
 fn main() {
     // Create a new LED attached to Pin 17
     let mut led = LED::new(17);
-    // blink the LED
-    led.blink(2.0, 3.0);
+
+    // on_time = 2 secs, off_time=3 secs
+    led.blink(2.0,3.0);
+
+    // prevent program from exiting immediately
+    led.wait();
 }