fix return value of getBytes (espressif#7981)

Author: supcik

Diff for: docs/source/api/preferences.rst

@@ -578,7 +578,7 @@ Copy a series of bytes stored against a given key in the currently open namespac
          - the number of bytes to be written to the buffer pointed to by ``buf``
 
    **Returns**
-      * if successful, the number of bytes equal to ``len`` is written to buffer ``buf``, and the method returns ``1``.
+      * if successful, the number of bytes equal to ``len`` is written to buffer ``buf``, and the method returns ``len``.
       * if the method fails, nothing is written to the buffer and the method returns ``0``.
 
    **Notes**
@@ -698,4 +698,4 @@ Get the number of free entries available in the key table of the currently open
       * A message providing the reason for a failed call is sent to the arduino-esp32 ``log_e`` facility.
 
 
-..    --- EOF ----
+..    --- EOF ----