net/textproto: replace '3-digit' with 'three-digit'
net/textproto: replace '3-digit' with 'three-digit'

A matter on form in documentation.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13087043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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