typo

cores/esp8266/WString.h

@@ -105,7 +105,7 @@ class String {
  String &operator =(const __FlashStringHelper *str);
  String &operator =(String &&rval) noexcept;
  String &operator =(char c) {
- char buf[2] { c, '\0' };
+ char buffer[2] { c, '\0' };
  *this = buffer;
  return *this;
  }