Hello
I'm not sure which style of JA ACM module you're using so assume that you have style-4, you can edit the XML file:
ROOT/templates/your_template/acm/features-intro/tmpl/style-4.xml
<?xml version="1.0" encoding="utf-8"?>
<form>
<title>Features Intro Style 4</title>
<sampledata></sampledata>
<fields name="features-intro">
<fieldset name="basic" label="FEATURES_BASIC_LABEL" description="FEATURES_BASIC_DESC">
<field name="intro-img" type="media" default="" label="FEATURES_INTRO_IMAGE_LABEL" description="FEATURES_INTRO_IMAGE_DESC" />
<field name="description" type="editor" default="" label="FEATURES_DESCRIPTION_LABEL" description="FEATURES_DESCRIPTION_DESC" rows="5" cols="40" filter="raw" />
<field name="desc-img" type="media" default="" label="FEATURES_DESC_MEDIA_LABEL" description="FEATURES_DESC_MEDIA_DESC" />
</fieldset>
</fields>
</form>
change the type="editor" for the description field as above.