Fix definition of `data-mx-color` values (#1260)

* Fix definition of `data-mx-color` values

* Create 1260.clarification

* Update changelogs/client_server/newsfragments/1260.clarification

Co-authored-by: Travis Ralston <travpc@gmail.com>

Co-authored-by: Travis Ralston <travpc@gmail.com>

changelogs/client_server/newsfragments/1260.clarification

@@ -0,0 +1 @@
+Fix various typos throughout the specification.

content/client-server-api/modules/instant_messaging.md

@@ -47,8 +47,9 @@ Not all attributes on those tags should be permitted as they may be
 avenues for other disruption attempts, such as adding `onclick` handlers
 or excessively large text. Clients should only permit the attributes
 listed for the tags below. Where `data-mx-bg-color` and `data-mx-color`
-are listed, clients should translate the value (a 6-character hex color
-code) to the appropriate CSS/attributes for the tag.
+are listed, clients should translate the value (a `#` character followed
+by a 6-character hex color code) to the appropriate CSS/attributes for
+the tag.
 
 `font`
 `data-mx-bg-color`, `data-mx-color`, `color`