doc: guidelines: fix title level

Specialized driver requirements should have been one level up, it now
nests under "treewide".

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

doc/contribute/guidelines.rst

@@ -1028,7 +1028,7 @@ support for the old one is not a treewide change. Deprecation and removal of
 such APIs, however, are treewide changes.
 
 Specialized driver requirements
-===============================
+*******************************
 
 Drivers for standalone devices should use the Zephyr bus APIs (SPI, I2C...)
 whenever possible so that the device can be used with any SoC from any vendor