doc: fix duplicate "this" and "the" on doc/api/http2.md

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Sending this PR from the NodeFest 2018 Tokyo 🇯🇵 code-and-learn class.