docs: Fixing bad summary fragment in Canvas.isOpaque()

The first sentence of the Canvas.isOpaque() comment contained an "i.e.",
which the docs engine interprets as the end of the summary fragment (per
the Javadoc standard). Replaced "i.e." with "that is" to fix this.

Docs-only change, no code affected. NO_TEST , fixes a doc bug.

Change-Id: Ib733365cc8654acdaf1d9b5bc891e0aa67210bf1
Test: [go/abtd docs build]
Fixes: 267544675