add optional to title of security email page

src/registrar/forms/domain.py

@@ -223,7 +223,7 @@ class DomainSecurityEmailForm(forms.Form):
     """Form for adding or editing a security email to a domain."""
 
     security_email = forms.EmailField(
-        label="Security email (optional)",
+        label="Security email",
         required=False,
         error_messages={
             "invalid": str(SecurityEmailError(code=SecurityEmailErrorCodes.BAD_DATA)),

src/registrar/templates/domain_security_email.html

@@ -6,7 +6,7 @@
 {% block domain_content %}
   {% include "includes/form_errors.html" with form=form %}
 
-  <h1>Security email</h1>
+  <h1>Security email (optional)</h1>
 
   <p>We strongly recommend that you provide a security email. This email will allow the  public to report observed or suspected security issues on your domain. Security emails are made public and included in the <a class="usa-link" rel="noopener noreferrer" target="_blank" href="{% public_site_url 'about/data/' %}">.gov domain data</a> we provide.</p>