internal/godoc/codec: replace braces in templates
internal/godoc/codec: replace braces in templates

The default text/template delimiters, {{ and }}, are
hard to distinguish when used for Go code. Change them
to guillemets, which don't look at all like code.

Change-Id: I0f8824e1ba5ab441d5a7e353bc7f0b8ac1298029
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/264258
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite