extensions: fix minor aesthetics

Author: Pablo Panero

docs/extensions/custom.md

@@ -61,4 +61,4 @@ $ invenio-cli run
 ```
 
 !!! note "UI related extensions"
-    if your extension adds scss and/or javascript, you will need to update your final static files before running your instance! You can do so with the CLI: `invenio-cli update [--no-install-js]`
+    If your extension adds scss and/or javascript, you will need to update your final static files before running your instance! You can do so with the CLI: `invenio-cli update [--no-install-js]`

docs/extensions/index.md

@@ -8,6 +8,6 @@ If you want to add custom functionality to your RDM instance, you need to develo
 
 [> Develop an extension](custom.md)
 
-### Storage
+## Storage
 
 - [S3](s3.md) can be configured to serve as your storage backend via the [invenio-s3](https://github.com/inveniosoftware/invenio-s3) extension. If you chose S3 as storage when initializing your instance, [navigate here](#i-chose-s3-when-initializing-the-repository). Otherwise, [navigate to this section](#i-didnt-choose-s3-when-initializing).