lib: stm32wba: README.rst rework

Rework the README.rst file to fix indentation issues.

(cherry picked from commit 50fcec604b9994f67cace4b4d47ab6c6048c9ac7)

Original-Signed-off-by: Vincent Tardy <vincent.tardy@st.com>
GitOrigin-RevId: 50fcec604b9994f67cace4b4d47ab6c6048c9ac7
Cr-Build-Id: 8696937592868157953
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8696937592868157953
Copybot-Job-Name: zephyr-hal_stm32-copybot-downstream
Change-Id: I42bd8c7cb72d623c7391f47636a48d7032508710
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/hal_stm32/+/7209459
Commit-Queue: ChromeOS Copybot <copybot@chops-service-accounts.iam.gserviceaccount.com>
Tested-by: ChromeOS Copybot <copybot@chops-service-accounts.iam.gserviceaccount.com>
Bot-Commit: ChromeOS Copybot <copybot@chops-service-accounts.iam.gserviceaccount.com>