pw_span: Fix typo and example in docs
pw_span: Fix typo and example in docs

Change-Id: I531e9f044ee81cf9b67206e1bf957af0b25d8179

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