Fix documentation on default build image name (change to correct separator).

[sebastian-correa]

What I did
This PR changes old documentation showing the old _ separator for image names.

Details:
150fd4b changed the default separator from _ to -, as seen in

compose/pkg/api/api.go

Lines 622 to 631 in 86cd523

	var Separator = "-"
	// GetImageNameOrDefault computes the default image name for a service, used to tag built images
	func GetImageNameOrDefault(service types.ServiceConfig, projectName string) string {
	imageName := service.Image
	if imageName == "" {
	imageName = projectName + Separator + service.Name
	}
	return imageName
	}

but the compose build docs still mention _:

Will this propagate downstream to docker/docs or do I need to submit specific PRs there for the updates?

A picture of a cute animal, if possible in relation to what you did

This should be mandatory. Sorry it's not related... This is my cat Panko being dracula.

[ndeloof]

LGTM
@glours do you remember why you created #9687 ? What's the drawback using underscore separator in image names ?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.68%. Comparing base (86cd523) to head (3a26f81).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11635      +/-   ##
==========================================
- Coverage   57.72%   57.68%   -0.05%     
==========================================
  Files         138      138              
  Lines       11736    11736              
==========================================
- Hits         6775     6770       -5     
- Misses       4284     4288       +4     
- Partials      677      678       +1     

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[sebastian-correa]

@ndeloof I think the change was made because compose v2 prefers the - separator so that service names are DNS valid. I know this has nothing to do with the image itself, but it was probably better (for consistency) and easier to have just 1 separator for everything.