doc: wemos: fix ReST underline

Use `=` sign instead of `-` to adhere to doc guidelines

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

boards/wemos/esp32s2_lolin_mini/doc/index.rst

@@ -31,15 +31,15 @@ Hardware
 ********
 
 Supported Features
-------------------
+==================
 
 .. zephyr:board-supported-hw::
 
 System requirements
 *******************
 
 Prerequisites
--------------
+=============
 
 Espressif HAL requires WiFi and Bluetooth binary blobs in order work. Run the command
 below to retrieve those files.
@@ -53,7 +53,7 @@ below to retrieve those files.
    It is recommended running the command above after :file:`west update`.
 
 Building & Flashing
--------------------
+===================
 
 Build and flash applications as usual (see :ref:`build_an_application` and
 :ref:`application_run` for more details).