http2: fix truncated comment

Backtick is my screen metakey, so this got lost in a copy/paste
at the backtick symbol.

Change-Id: Ia3e7bf5d2819ea83ff6cc86cc3b6210e3f279eb0
Reviewed-on: https://go-review.googlesource.com/21193
Reviewed-by: David Symonds <dsymonds@golang.org>

http2/http2.go

@@ -170,8 +170,9 @@ var (
 //  RFC 7230 says:
 //   header-field   = field-name ":" OWS field-value OWS
 //   field-name     = token
+//   token          = 1*tchar
 //   tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
-//           "^" / "_" / "
+//           "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
 // Further, http2 says:
 //   "Just as in HTTP/1.x, header field names are strings of ASCII
 //   characters that are compared in a case-insensitive