alt-svc: use of "Alt-Svc header field" term before it has been introduced

[LPardue]

Section 2.2

Mechanisms for discovering alternative services also associate a freshness lifetime with them; for example, the Alt-Svc header field uses the "ma" parameter.

The term Alt-Svc header field has not been introduced at this stage. It is indicated in Section 2 This document describes two such mechanisms: an HTTP header field..., however it may help to name it explicitly there, or somewhere before Section 2.2.