pw_sys_io_baremetal_stm32f42: Fix kReadDataReady comment
pw_sys_io_baremetal_stm32f42: Fix kReadDataReady comment

Correct the comment for this constant to correctly identify
the structure member to which it applies.

Change-Id: Ibef80da2f0c1a45bf379eb8bb7f7900414c41d20
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/118570
Commit-Queue: Chris Mumford <86e3bd41c0fc7139fc638a34b6b342d4b7b77b85@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed