Fix preamble it is required not optional (#5089)

Author: swoboda1337

components/sx127x.rst

@@ -61,7 +61,7 @@ LoRa configuration variables:
   packet size is fixed. If not configured explicit header mode is enabled and variable packet sizes can
   be used. Maximum length is 256. Must be greater than zero when using a ``spreading_factor`` of 6.
 - **crc_enable** (**Optional**, bool): Enables a payload CRC calculation/check.
-- **preamble_size** (**Optional**, int): Length of the preamble in symbols, minimum of 6. Defaults to 8.
+- **preamble_size** (**Required**, int): Length of the preamble in symbols, minimum of 6.
 - **spreading_factor** (**Optional**, int): Spreading factor, values range from 6 to 12. Defaults to 7.
 - **coding_rate** (**Optional**, enum): Coding rate, values can be ``CR_4_5``, ``CR_4_6``, ``CR_4_7``
   or ``CR_4_8``. Defaults to ``CR_4_5``.