Update WString.h (#1935)

Author: terryjmyers

Diff for: cores/esp32/WString.h

@@ -260,6 +260,7 @@ class String
     // parsing/conversion
     long toInt(void) const;
     float toFloat(void) const;
+    double toDouble(void) const;
 
 protected:
     char *buffer;	        // the actual char array