Update CMakeLists.txt (#78)

Correct STM3F7xx to STM32F7xx in order to compile correctly

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Description
-----------
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The FREERTOS_PLUS_FAT_PORT option was wrong in CMakeList.txt missing a
"2" in order to align to other part of code

Test Steps
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Try to compile with option

set(FREERTOS_PLUS_FAT_PORT "STM32F7XX" CACHE STRING "" FORCE)
in originale file and you will have a error becaus you don't compile .c
file

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- [X] I have tested my changes. No regression in existing tests.
- [ ] I have modified and/or added unit-tests to cover the code changes
in this Pull Request.

Related Issue
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Author: micheledarold

portable/CMakeLists.txt

@@ -54,7 +54,7 @@ target_sources( freertos_plus_fat_port
         STM32F4xx/stm32f4xx_hal_sd.h
         STM32F4xx/stm32f4xx_ll_sdmmc.c
         STM32F4xx/stm32f4xx_ll_sdmmc.h>
-    $<$<STREQUAL:${FREERTOS_PLUS_FAT_PORT},STM3F7XX>:
+    $<$<STREQUAL:${FREERTOS_PLUS_FAT_PORT},STM32F7XX>:
         STM32F7xx/ff_sddisk.c
         STM32F7xx/stm32f7xx_hal_sd.c
         STM32F7xx/stm32f7xx_hal_sd.h>