Use LED_BUILTIN so that it works w/o attaching external LED (#3452)

* Use LED_BUILTIN so that it works w/o attaching external LED

* Use built-in LED

* Clarify text

Author: probonopd

libraries/Ticker/examples/TickerBasic/TickerBasic.ino

@@ -9,8 +9,7 @@
   There are two variants of the attach function: attach and attach_ms.
   The first one takes period in seconds, the second one in milliseconds.
   
-  An LED connected to GPIO1 will be blinking. Use a built-in LED on ESP-01
-  or connect an external one to TXD on other boards.
+  The built-in LED will be blinking.
 */
 
 #include <Ticker.h>
@@ -21,8 +20,8 @@ int count = 0;
 
 void flip()
 {
-  int state = digitalRead(1);  // get the current state of GPIO1 pin
-  digitalWrite(1, !state);     // set pin to the opposite state
+  int state = digitalRead(LED_BUILTIN);  // get the current state of GPIO1 pin
+  digitalWrite(LED_BUILTIN, !state);     // set pin to the opposite state
 
   ++count;
   // when the counter reaches a certain value, start blinking like crazy
@@ -38,8 +37,8 @@ void flip()
 }
 
 void setup() {
-  pinMode(1, OUTPUT);
-  digitalWrite(1, LOW);
+  pinMode(LED_BUILTIN, OUTPUT);
+  digitalWrite(LED_BUILTIN, LOW);
 
   // flip the pin every 0.3s
   flipper.attach(0.3, flip);

libraries/Ticker/examples/TickerParameter/TickerParameter.ino

@@ -8,8 +8,7 @@
   This sample runs two tickers that both call one callback function,
   but with different arguments.
 
-  An LED connected to GPIO1 will be pulsing. Use a built-in LED on ESP-01
-  or connect an external one to TXD on other boards.
+  The built-in LED will be pulsing.
 */
 
 #include <Ticker.h>
@@ -18,11 +17,11 @@ Ticker tickerSetHigh;
 Ticker tickerSetLow;
 
 void setPin(int state) {
-  digitalWrite(1, state);
+  digitalWrite(LED_BUILTIN, state);
 }
 
 void setup() {
-  pinMode(1, OUTPUT);
+  pinMode(LED_BUILTIN, OUTPUT);
   digitalWrite(1, LOW);
 
   // every 25 ms, call setPin(0)