Update set.md

Added string example, It needs a single quote inside the double quote. to read a string. Otherwise, it was giving compilation error. Set is heavily used as a variable alternative.

website/docs/reference/dbt-jinja-functions/set.md

@@ -27,6 +27,10 @@ __Args__:
 {% do log(my_set) %}  {# None #}
 ```
 
+```
+{% set email_id = "'[email protected]'" %}
+```
+
 ### set_strict
 
 The `set_strict` context method can be used to convert any iterable to a sequence of iterable elements that are unique (a set). The difference to the `set` context method is that the `set_strict` method will raise an exception on a `TypeError`, if the provided value is not a valid iterable and cannot be converted to a set.