Fix a typo in STM32F3's CMakeList.txt

[douardda]

A typo has been introduced preventing the STM32F303x8 to be buildable using the new cmake based build system.

Summary of changes

Fix a typo in STM32F3's CMakeList.txt file

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[0xc0170]

Thanks @douardda

[0xc0170]

CI started

[mbed-ci]

Jenkins CI Test : ✔️ SUCCESS

Build Number: 1 | 🔒 Jenkins CI Job | 🌐 Logs & Artifacts

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jobs	Status
jenkins-ci/mbed-os-ci_unittests	✔️
jenkins-ci/mbed-os-ci_cmake-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_cmake-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-cloud-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-greentea-ARM	✔️
jenkins-ci/mbed-os-ci_build-cloud-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-greentea-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_dynamic-memory-usage	✔️
jenkins-ci/mbed-os-ci_greentea-test	✔️
jenkins-ci/mbed-os-ci_cmake-example-test	✔️
jenkins-ci/mbed-os-ci_cloud-client-pytest	✔️