Documentation: moved 'ESP32 Compatibility' to subsection (#2311)

Lowered level of segment 'ESP32 Compatibility' in documentation of 'WiFiClientSecure Class' to be a subsection thereof.

Author: djpearman

docs/bearssl-client-secure-class.rst

@@ -120,7 +120,7 @@ Valid values for min and max are `BR_TLS10`, `BR_TLS11`, `BR_TLS12`.  Min and ma
 
 
 ESP32 Compatibility
-===================
+~~~~~~~~~~~~~~~~~~~
 Simple ESP32 ``WiFiClientSecure`` compatibility is built-in, allow for some sketches to run without any modification.
 The following methods are implemented: