doc: fix e.g., to e.g. in doc/http.md

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

• doc

Description of change

doc: fix e.g., to e.g. in BUILDING.md