Fix rendering of external groups in READMEs

Documentations rendering was looking for fields in the built directory,
there the external fields have been already resolved and they don't
include the external key.

For legacy reasons, elastic-package only renders groups in READMEs when
they are external, so these fields were not being rendered.

Author: jsoriano

internal/docs/readme.go

@@ -131,7 +131,7 @@ func UpdateReadmes(repositoryRoot *os.Root, packageRoot, packageBuildRoot string
 func updateReadme(fileName, linksFilePath, packageRoot, packageBuildRoot string) (string, error) {
 	logger.Debugf("Update the %s file", fileName)
 
-	rendered, shouldBeRendered, err := generateReadme(fileName, linksFilePath, packageRoot, packageBuildRoot)
+	rendered, shouldBeRendered, err := generateReadme(fileName, linksFilePath, packageBuildRoot, packageRoot)
 	if err != nil {
 		return "", err
 	}