docs: fix typo in url fragment
I noticed the broken hyperlink in this page:
The link should point to `#parser-directives`.
@@ -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
