Make lint annotations warnings (mdn#19868)

Author: OnkarRuikar

.github/workflows/markdownlint-problem-matcher.json

@@ -2,6 +2,7 @@
   "problemMatcher": [
     {
       "owner": "markdownlint",
+      "severity": "warning",
       "pattern": [
         {
           "regexp": "^([^:]*):(\\d+):?(\\d+)?\\s([\\w-\\/]*)\\s(.*)$",

files/en-us/web/css/border/index.md

@@ -17,7 +17,7 @@ The **`border`** [shorthand](/en-US/docs/Web/CSS/Shorthand_properties) [CSS](/en
 
 {{EmbedInteractiveExample("pages/css/border.html")}}
 
-## Constituent properties
+## Constituent properties 
 
 This property is a shorthand for the following CSS properties: