i2c is driven from system_clock, noch peripheral_clock. Update example accordingly.

rp2040-hal/examples/i2c.rs

@@ -89,7 +89,7 @@ fn main() -> ! {
         scl_pin, // Try `not_an_scl_pin` here
         400.kHz(),
         &mut pac.RESETS,
-        &clocks.peripheral_clock,
+        &clocks.system_clock,
     );
 
     // Write three bytes to the I²C device with 7-bit address 0x2C