docs: fix typo in url fragment

I noticed the broken hyperlink in this page:
https://docs.docker.com/engine/reference/builder/
The link should point to `#parser-directives`.

Signed-off-by: Mihai Borobocea <MihaiBorob@gmail.com>

docs/reference/builder.md

@@ -120,7 +120,7 @@ instruction must be \`FROM\`** in order to specify the [*Base
 Image*](glossary.md#base-image) from which you are building.
 
 Docker treats lines that *begin* with `#` as a comment, unless the line is
-a valid [parser directive](builder.md#parser directives). A `#` marker anywhere
+a valid [parser directive](builder.md#parser-directives). A `#` marker anywhere
 else in a line is treated as an argument. This allows statements like:
 
 ```Dockerfile