Update format strings in docs/examples for correct signedness.

The latest version of Clang 19 detects sign mismatches in format
strings when -Wformat is enabled.

Change-Id: I2b1516dff8f636a831ff6566b8382bf6b29e1203
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/842477
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
7 files changed