Link horizontal whitespace to the grammar rule

The prose mentions "horizontal whitespace" in two places (fence
trailing content and infostring trailing content) without linking
to the grammar definition.  Since `HORIZONTAL_WHITESPACE` is now a
defined rule in Whitespace, let's add a link so readers can click
through to the precise definition.

Author: traviscross

src/frontmatter.md

@@ -54,14 +54,15 @@ r[frontmatter.document]
 Frontmatter may only be preceded by a [shebang] and [whitespace].
 
 r[frontmatter.fence]
-Frontmatter must start and end with a *fence*. Each fence must start at the beginning of a line. The opening fence must consist of at least 3 and no more than 255 hyphens (`-`). The closing fence must have exactly the same number of hyphens as the opening fence. The hyphens of either fence may be followed by horizontal whitespace.
+Frontmatter must start and end with a *fence*. Each fence must start at the beginning of a line. The opening fence must consist of at least 3 and no more than 255 hyphens (`-`). The closing fence must have exactly the same number of hyphens as the opening fence. The hyphens of either fence may be followed by [horizontal whitespace].
 
 r[frontmatter.infostring]
-The opening fence, after optional horizontal whitespace, may be followed by an infostring that identifies the format or purpose of the body. An infostring may be followed by horizontal whitespace.
+The opening fence, after optional [horizontal whitespace], may be followed by an infostring that identifies the format or purpose of the body. An infostring may be followed by horizontal whitespace.
 
 r[frontmatter.body]
 No line in the body may start with a sequence of hyphens (`-`) equal to or longer than the opening fence. The body may not contain any carriage returns (that survive [CRLF normalization]).
 
 [CRLF normalization]: input.crlf
+[horizontal whitespace]: grammar-HORIZONTAL_WHITESPACE
 [shebang]: input-format.md#shebang-removal
 [whitespace]: whitespace.md