Fix impl Gpio -> impl GpioConfig`

In the two sections "Peripherals as State Machines" and "Design
Contracts" of the "Static Guarantees" chapter, `impl Gpio` was
incorrectly used in place of `impl GpioConfig` at one place in each
section.

This commit fixes that.

src/static-guarantees/design-contracts.md

@@ -25,7 +25,7 @@ struct GpioConfig {
     periph: GPIO_CONFIG,
 }
 
-impl Gpio {
+impl GpioConfig {
     pub fn set_enable(&mut self, is_enabled: bool) {
         self.periph.modify(|_r, w| {
             w.enable().set_bit(is_enabled)

src/static-guarantees/state-machines.md

@@ -60,7 +60,7 @@ struct GpioConfig {
     periph: GPIO_CONFIG,
 }
 
-impl Gpio {
+impl GpioConfig {
     pub fn set_enable(&mut self, is_enabled: bool) {
         self.periph.modify(|_r, w| {
             w.enable().set_bit(is_enabled)