content/constants: fix display of two backquote characters

Two backquote characters were expressed as 6 consecutive backquotes in
the legacy article format. That unusual sequence didn't translate well
into the new Markdown article format in CL 222848.
Replace it with simple HTML, since this is hard to express in Markdown.

For golang/go#33955.

Change-Id: I575aa89c9f9104de694d9a8dad4ca103cc4c1567
Reviewed-on: https://go-review.googlesource.com/c/blog/+/231817
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/content/constants.article b/content/constants.article
index 23e77fc..51308d6 100644
--- a/content/constants.article
+++ b/content/constants.article
@@ -92,7 +92,7 @@
 to explore the type issues of constants in Go.
 
 A string constant encloses some text between double quotes.
-(Go also has raw string literals, enclosed by backquotes ````````,
+(Go also has raw string literals, enclosed by backquotes <code>``</code>,
 but for the purpose of this discussion they have all the same properties.)
 Here is a string constant: