fix(html): Update form validation phone solution to work with v flag (#725)

The current version does not work, this is because of the change to
using the v flag for regex requiring - to be escaped.

https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/pattern

Author: Brian Lister

html/forms/tasks/form-validation/marking.md

@@ -47,7 +47,7 @@ Now we want you to take the same form you saw in the previous task (use your pre
 
 1. All of the user names in our application consist of a single letter, followed by a dot, followed by three or more letters or numbers. All letters should be lowercase. This will work — `pattern="[a-z]{1}\.[a-z0-9]{3,}"`.
 2. All of the email addresses for our users consist of one or more letters (lower or upper case) or numbers, followed by "@bigcorp.eu". This will work — `[a-zA-Z0-9]+@bigcorp\.eu`.
-3. Remove the length validation from the phone number field if it is present, and set it so that it accepts 10 digits — either 10 digits in a row, or a pattern of three digits, three digits, then four digits, separated by either spaces, dashes, or dots. The following with work for this — `pattern="[0-9]{10}|[0-9]{3}[-. ][0-9]{3}[-. ][0-9]{4}"` — although bear in mind that it will work if the separators are not all the same (e.g. one dot and two dashes). This is OK for the purposes of this task — you'd be unlikely to get people entering numbers like this anyway, and it would be easy to sanitize the data on the server.
+3. Remove the length validation from the phone number field if it is present, and set it so that it accepts 10 digits — either 10 digits in a row, or a pattern of three digits, three digits, then four digits, separated by either spaces, dashes, or dots. The following with work for this — `pattern="[0-9]{10}|[0-9]{3}[\-. ][0-9]{3}[\-. ][0-9]{4}"` — although bear in mind that it will work if the separators are not all the same (e.g. one dot and two dashes). This is OK for the purposes of this task — you'd be unlikely to get people entering numbers like this anyway, and it would be easy to sanitize the data on the server.
 
 Try submitting your form — it should refuse to submit until the above constraints are followed, and give suitable error messages. To help, you might want to consider adding some simple CSS to show when a form field is valid or invalid.
 
@@ -70,7 +70,7 @@ The finished markup should look something like this:
     <li>
       <label for="phone">Phone number:</label>
       <input type="text" name="phone" id="phone" required
-             pattern="[0-9]{10}|[0-9]{3}[-. ][0-9]{3}[-. ][0-9]{4}">
+             pattern="[0-9]{10}|[0-9]{3}[\-. ][0-9]{3}[\-. ][0-9]{4}">
     </li>
     <li>
       <label for="comment">Comment:</label>