Fixed formatting typo in --purge-only command flag

Was rendering as `-purge-only` (with one dash)
Fixes #11646

CHANGELOG.txt

@@ -22,6 +22,7 @@ Changelog
  * Docs: Make sure the settings panel is listed in tabbed interface examples (Tibor Leupold)
  * Docs: Update content and page names to their US spelling instead of UK spelling (Victoria Poromon)
  * Docs: Update broken and incorrect links throughout the documentation (EK303)
+ * Docs: Fix formatting of `--purge-only` in `wagtail_update_image_renditions` management command section (Pranith)
  * Maintenance: Move RichText HTML whitelist parser to use the faster, built in `html.parser` (Jake Howard)
  * Maintenance: Remove duplicate 'path' in default_exclude_fields_in_copy (Ramchandra Shahi Thakuri)
  * Maintenance: Update unit tests to always use the faster, built in `html.parser` & remove `html5lib` dependency (Jake Howard)

CONTRIBUTORS.md

@@ -803,6 +803,7 @@
 * EK303
 * Damian Borneman
 * Viktor Szépe
+* Pranith
 
 ## Translators
 

docs/reference/management_commands.md

@@ -162,5 +162,5 @@ This does not remove unused rendition images, this can be done by clearing the f
 
 Options:
 
--   **--purge-only** :
+-  `--purge-only` :
     This argument will purge all image renditions without regenerating them. They will be regenerated when next requested.

docs/releases/6.1.md

@@ -39,6 +39,7 @@ depth: 1
  * Make sure the settings panel is listed in tabbed interface examples (Tibor Leupold)
  * Update content and page names to their US spelling instead of UK spelling (Victoria Poromon)
  * Update broken and incorrect links throughout the documentation (EK303)
+ * Fix formatting of `--purge-only` in [`wagtail_update_image_renditions`](wagtail_update_image_renditions) management command section (Pranith)
 
 
 ### Maintenance