fix: 修改GPIO中的例子错误

Author: HalfSweet

docs/library/gpio.md

@@ -116,21 +116,22 @@ void detachInterrupt(uint32_t channel)
 ### GPIO 输入和输出模式
 
 ```cpp
-const auto LED = PB0;
+const auto ledPin = PB0;
 const auto BUTTON = PF4;
 
 uint8_t stateLED = 0;
 
 void setup() {
-    pinMode(LED, OUTPUT);
+    pinMode(ledPin, OUTPUT);
     pinMode(BUTTON,INPUT_PULLUP);
 }
 
 void loop() {
-   if(!digitalRead(BUTTON)){
-     stateLED = stateLED^1;
-    digitalWrite(LED,stateLED);
-  }
+   if(!digitalRead(BUTTON))
+    {
+        stateLED = stateLED^1;
+        digitalWrite(ledPin,stateLED);
+    }
 }
 ```