Replace quotation marks with backticks

Author: ludwig031

doc/CAPTCHA.md

@@ -228,6 +228,6 @@ Or, using invisible captcha:
 {% endblock %}
 ```
 
-If captcha validation results in error with code "missing-input-response" you probably did not send reCAPTCHA token in variable "g-recaptcha-response".
-Another error you might encounter is "action-mismatch". In this case make sure that the action you specified in captcha config (globally or per content type)
+If captcha validation results in error with code `missing-input-response` you probably did not send reCAPTCHA token in variable `g-recaptcha-response`.
+Another error you might encounter is `action-mismatch`. In this case make sure that the action you specified in captcha config (globally or per content type)
  is the same as the one you use when executing captcha check.