Documentation - clean up last two internal refs to use underscore

- As per https://docs.wagtail.org/en/latest/contributing/documentation_guidelines.html#reference-links - refs should be `lower_snake_case`

docs/advanced_topics/customisation/page_editing_interface.md

@@ -29,7 +29,7 @@ class BlogPage(Page):
     ])
 ```
 
-(rich-text)=
+(rich_text)=
 
 ## Rich Text (HTML)
 

docs/extending/rich_text_internals.md

@@ -15,7 +15,7 @@ The components involved in Wagtail's rich text handling are described below.
 
 ## Data format
 
-Rich text data (as handled by [RichTextField](rich-text), and `RichTextBlock` within [StreamField](../topics/streamfield.rst)) is stored in the database in a format that is similar, but not identical, to HTML. For example, a link to a page might be stored as:
+Rich text data (as handled by [RichTextField](rich_text), and `RichTextBlock` within [StreamField](../topics/streamfield.rst)) is stored in the database in a format that is similar, but not identical, to HTML. For example, a link to a page might be stored as:
 
 ```html
 <p><a linktype="page" id="3">Contact us</a> for more information.</p>

docs/reference/pages/panels.md

@@ -1,4 +1,4 @@
-(editing-api)=
+(editing_api)=
 
 # Panel types
 

docs/reference/panel_api.md

@@ -4,7 +4,7 @@
 .. module:: wagtail.admin.panels
 ```
 
-This document describes the reference API for the base `Panel` and the `BoundPanel` classes that are used to render Wagtail's panels. For available panel types and how to use them, see [](editing-api).
+This document describes the reference API for the base `Panel` and the `BoundPanel` classes that are used to render Wagtail's panels. For available panel types and how to use them, see [](editing_api).
 
 ## `Panel`
 

docs/topics/images.md

@@ -294,7 +294,7 @@ And to use the `<amp-img>` tag (based on the [Mountains example](https://amp.dev
 
 ## Images embedded in rich text
 
-The information above relates to images defined via image-specific fields in your model. However, images can also be embedded arbitrarily in Rich Text fields by the page editor (see [](rich-text)).
+The information above relates to images defined via image-specific fields in your model. However, images can also be embedded arbitrarily in Rich Text fields by the page editor (see [](rich_text)).
 
 Images embedded in Rich Text fields can't be controlled by the template developer as easily. There are no image objects to work with, so the `{% image %}` template tag can't be used. Instead, editors can choose from one of a number of image "Formats" at the point of inserting images into their text.