field/@as-type="markup-line" needs to be classified as markup

Otherwise, we would not be able to parse contents like

            <param id="ac-4.4_prm_2">
               <select how-many="one or more">
                  <choice>decrypting the information</choice>
                  <choice>blocking the flow of the encrypted information</choice>
                  <choice>terminating communications sessions attempting to pass encrypted information</choice>
                  <choice>
                     <insert param-id="ac-4.4_prm_3"/>
                  </choice>
               </select>
            </param>

that appears in NIST_SP-800-53_rev5-FINAL_catalog.xml

metaschema/parser/main.go

@@ -168,7 +168,7 @@ func (df *DefineField) GetMetaschema() *Metaschema {
 }
 
 func (df *DefineField) IsMarkup() bool {
-	return df.AsType == AsTypeMarkupMultiLine
+	return df.AsType == AsTypeMarkupMultiLine || df.AsType == AsTypeMarkupLine
 }
 
 func (df *DefineField) JsonName() string {