zephyr: cmake: Fix a typo in file CMakeLists.txt

Replace "bur" with "but".

Signed-off-by: Ding Tao <miyatsu@qq.com>
diff --git a/boot/zephyr/CMakeLists.txt b/boot/zephyr/CMakeLists.txt
index 318ca48..4ce1a8b 100644
--- a/boot/zephyr/CMakeLists.txt
+++ b/boot/zephyr/CMakeLists.txt
@@ -113,7 +113,7 @@
 Try adding CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256=y to your prj.conf, \
 delete the build folder and build from scratch.")
   endif()
-  # Since here we are not using Zephyr's mbedTLS bur rather our own, we need
+  # Since here we are not using Zephyr's mbedTLS but rather our own, we need
   # to set MBEDTLS_CONFIG_FILE ourselves. When using Zephyr's copy, this
   # variable is set by its Kconfig in the Zephyr codebase.
   target_compile_definitions(app PRIVATE MBEDTLS_CONFIG_FILE="${CMAKE_CURRENT_LIST_DIR}/include/mcuboot-mbedtls-cfg.h")