ibmButton attribute value (#3076)

ibmButton attribute (directive) needs to value. According the page preview at the top I add `primary`.

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

src/pages/developing/angular-tutorial/step-2.mdx

@@ -390,7 +390,7 @@ We can now add our images and text for each column in the first `Tab` in
           design tools and resources, human interface guidelines,
           and a vibrant community of contributors.
         </p>
-        <button ibmButton>Learn more</button>
+        <button ibmButton="primary">Learn more</button>
       </div>
       <div ibmCol class="cds--offset-lg-1" [columnNumbers]="{'md': 4, 'lg': 8}">
         <img