[libc++][format][chrono] Addresses LWG3842.

  LWG3842 Unclear wording for precision in chrono-format-spec

Note there is nothing to do, the issue clarifies the wording in the Standard.
The new wording matches my interpretation of the previous wording and this has
already been implemented in libc++.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D144328