doc: releases: fix kconfig role typo in mbedtls section

trivial typo fix where single backticks should be used instead of double
for the sphinx role kconfig:option

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

Author: kartben

doc/releases/migration-guide-4.0.rst

@@ -48,8 +48,8 @@ Mbed TLS
 * The Kconfig options ``CONFIG_MBEDTLS_TLS_VERSION_1_0`` and ``CONFIG_MBEDTLS_TLS_VERSION_1_1``
   have been removed because Mbed TLS doesn't support TLS 1.0 and 1.1 anymore since v3.0. (:github:`76833`)
 * The following Kconfig symbols were renamed (:github:`76408`):
-  * ``CONFIG_MBEDTLS_ENTROPY_ENABLED`` is now :kconfig:option:``CONFIG_MBEDTLS_ENTROPY_C``,
-  * ``CONFIG_MBEDTLS_ZEPHYR_ENTROPY`` is now :kconfig:option:``CONFIG_MBEDTLS_ENTROPY_POLL_ZEPHYR``.
+  * ``CONFIG_MBEDTLS_ENTROPY_ENABLED`` is now :kconfig:option:`CONFIG_MBEDTLS_ENTROPY_C`,
+  * ``CONFIG_MBEDTLS_ZEPHYR_ENTROPY`` is now :kconfig:option:`CONFIG_MBEDTLS_ENTROPY_POLL_ZEPHYR`.
 
 * The Kconfig option ``CONFIG_MBEDTLS_SSL_EXPORT_KEYS`` was removed because the
   corresponding build symbol was removed in Mbed TLS 3.1.0 and is now assumed to