Add %20 for spaces in mailto link

Use %20 for the space character to be more sure that different email clients will interpret this when opening the mailto link. Added to code block and brief explanation below for why to use it (could be rephrased if necessary).

Author: calumryan

html/lesson1/tutorial.md

@@ -297,10 +297,10 @@ Links can also open up a user's email client and share content. The difference b
 ```html
 <ul>
   <li>
-     <a href="mailto:[email protected]?subject=I love owls :: codebar">Email us</a>
+     <a href="mailto:[email protected]?subject=I%20love%20owls%20::%20codebar">Email us</a>
   </li>
   <li>
-     <a href="mailto:?subject=I love owls :: codebar">Email a friend</a>
+     <a href="mailto:?subject=I%20love%20owls%20::%20codebar">Email a friend</a>
   </li>
 </ul>
 ```
@@ -309,7 +309,9 @@ Links can also open up a user's email client and share content. The difference b
 
 > What happens when you click the second link? How is it different?
 
-> What happens when you add `&body=Owls are amazing` to the second link?
+> What happens when you add `&body=Owls%20are%20amazing` to the second link?
+
+Note that replacing spaces in the subject text with **%20** isn't essential but makes sure the spaces are kept when opening the link in a range of both new and older email clients/software.
 
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