text/template: improve comment example in doc

Make comment example consistent with its description.
Fixes #24767

Change-Id: Icff54b489040b1ce66c644a21343c6576304cf75
Reviewed-on: https://go-review.googlesource.com/107663Reviewed-by: Rob Pike <r@golang.org>

src/text/template/doc.go

@@ -69,6 +69,7 @@ data, defined in detail in the corresponding sections that follow.
 
 */
 //	{{/* a comment */}}
+//	{{- /* a comment with white space trimmed from preceding and following text */ -}}
 //		A comment; discarded. May contain newlines.
 //		Comments do not nest and must start and end at the
 //		delimiters, as shown here.