fix: normalize caption indentation in foundations.rst

The #0082C9 caption used tabs while the :alt: option used spaces,
causing Sphinx to reject it as a malformed figure caption.

AI-Assisted-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

developer_manual/design/foundations.rst

@@ -17,7 +17,7 @@ Primary color
 .. figure:: ../images/colour-primary.svg
    :alt: Nextcloud primary color swatch showing blue #0082C9
 
-		#0082C9
+   #0082C9
 
 While this is the primary color associated with Nextcloud and can be used to draw attention to an element, it is best to limit the usage of this to primary actions and other important elements.