docs: add anchor-tags for flags

This will generate "details_url" for options, so that the options
table on the pages at docs.docker.com link to the section describing
the option.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/plugin_ls.md

@@ -43,7 +43,7 @@ ID            NAME                                    DESCRIPTION
 69553ca1d123  tiborvass/sample-volume-plugin:latest   A test plugin for Docker   true
 ```
 
-### Filtering
+### <a name=format></a> Filtering (--filter)
 
 The filtering flag (`-f` or `--filter`) format is of "key=value". If there is more
 than one filter, then pass multiple flags (e.g., `--filter "foo=bar" --filter "bif=baz"`)
@@ -73,7 +73,7 @@ $ docker plugin ls --filter enabled=true
 ID                  NAME                DESCRIPTION         ENABLED
 ```
 
-### Formatting
+### <a name=format></a> Format the output (--format)
 
 The formatting options (`--format`) pretty-prints plugins output
 using a Go template.