Fix typo in index.md

doc/ci/introduction/index.md

@@ -156,7 +156,7 @@ Once you're happy with your implementation:
 
 ![GitLab workflow example](img/gitlab_workflow_example_11_9.png)
 
-GitLab CI/CD is capable of a doing a lot more, but this workflow
+GitLab CI/CD is capable of doing a lot more, but this workflow
 exemplifies GitLab's ability to track the entire process,
 without the need of any external tool to deliver your software.
 And, most usefully, you can visualize all the steps through