Avoid unnecessary calls in rewindDirectory reference code

The reference page for `File::rewindDirectory` provides a sketch demonstrating its usage.

This code calls the function after iterating over each directory on the card. This would be necessary if the
`printDirectory` function was configured to iterate over that directory multiple times, but it is not. So the calls are
redundant.

Moving the call to the `setup` function has the following benefits:

- Makes the use of the function more prominent. The reader can get an overview of what the program is doing without
  delving into the `printDirectory` function
- Avoids the misconception that it is required to call the function recursively as was done before

Author: per1234

docs/api.md

@@ -833,6 +833,7 @@ void setup() {
   SD.begin(10);
   root = SD.open("/");
   printDirectory(root, 0);
+  root.rewindDirectory();  // Return to the first file in the directory
   Serial.println("Done!");
 }
 
@@ -845,8 +846,6 @@ void printDirectory(File dir, int numTabs) {
      File entry =  dir.openNextFile();
      if (! entry) {
        // No more files
-       // Return to the first file in the directory
-       dir.rewindDirectory();
        break;
      }