doc: kernel: drivers: Correct number of initialization levels

Commit a0b746 deleted 2 deprecated driver init levels (EARLY,
APPLICATION), but didn't update the number mentioned in the doc.

Signed-off-by: Nabil Elqatib <nabilelqatib@gmail.com>

Author: n-eq

doc/kernel/drivers/index.rst

@@ -343,7 +343,7 @@ Initialization Levels
 Drivers may depend on other drivers being initialized first, or require
 the use of kernel services. :c:func:`DEVICE_DEFINE()` and related APIs
 allow the user to specify at what time during the boot sequence the init
-function will be executed. Any driver will specify one of four
+function will be executed. Any driver will specify one of three
 initialization levels:
 
 ``PRE_KERNEL_1``