text/template: fix documentation for pipelines

text/template: fix documentation for pipelines

The header was in the wrong place, so the definition of a pipeline
was not in the section labeled "Pipelines".

Fixes #13972

Change-Id: Ibca791a4511ca112047b57091c391f6e959fdd78
Reviewed-on: https://go-review.googlesource.com/18775
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>