Update offload optimize.md (#23415)

`###` should be `##`

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Author: tadashi0713

content/manuals/offload/optimize.md

@@ -29,7 +29,7 @@ setup for Docker Offload:
 
 For general Dockerfile tips, see [Building best practices](/manuals/build/building/best-practices.md).
 
-### dockerignore files
+## dockerignore files
 
 A [`.dockerignore` file](/manuals/build/concepts/context.md#dockerignore-files)
 lets you specify which local files should *not* be included in the build
@@ -45,15 +45,15 @@ Typical items to ignore:
 
 As a rule of thumb, your `.dockerignore` should be similar to your `.gitignore`.
 
-### Slim base images
+## Slim base images
 
 Smaller base images in your `FROM` instructions can reduce final image size and
 improve build performance. The [`alpine`](https://hub.docker.com/_/alpine) image
 is a good example of a minimal base.
 
 For fully static binaries, you can use [`scratch`](https://hub.docker.com/_/scratch), which is an empty base image.
 
-### Multi-stage builds
+## Multi-stage builds
 
 [Multi-stage builds](/build/building/multi-stage/) let you separate build-time
 and runtime environments in your Dockerfile. This not only reduces the size of
@@ -62,7 +62,7 @@ the final image but also allows for parallel stage execution during the build.
 Use `COPY --from` to copy files from earlier stages or external images. This
 approach helps minimize unnecessary layers and reduce final image size.
 
-### Fetch remote files in build
+## Fetch remote files in build
 
 When possible, download large files from the internet during the build itself
 instead of bundling them in your local context. This avoids network transfer
@@ -80,4 +80,4 @@ supports it, configure it to run in parallel. For example, use `make --jobs=4`
 to run four jobs simultaneously.
 
 Taking advantage of available CPU resources in the cloud can significantly
-improve build time.
+improve build time.