docs: remove space in img build spec docs (#2205)

## Description:


## Is this change user facing?
YES

docs/docs/api-reference/starlark-reference/image-build-spec.md

@@ -1,6 +1,6 @@
 ---
-title: Image Build Spec
-sidebar_label: Image Build Spec
+title: ImageBuildSpec
+sidebar_label: ImageBuildSpec
 ---
 
 Kurtosis starts services based on a provided image definition in the `image` arg of [`ServiceConfig`](./service-config.md). You can provide Kurtosis with a published image to use or alternatively, use `ImageBuildSpec` to instruct Kurtosis to build the Docker image the service will be started from.