Add missing word "you" (#453)

Signed-off-by: David A. Wheeler <[email protected]>

Author: david-a-wheeler

docs/labs/regex1.html

@@ -293,7 +293,7 @@ <h2>Background</h2>
 So <tt>[A-D]*</tt> will match 0 or more letters as long as every letter
 is an A, B, C, or D.
 <li>You can use "<tt>|</tt>" to identify options, any of which are acceptable.
-When validating input, should surround the collection of options
+When validating input, you should surround the collection of options
 with parenthesis, because "<tt>|</tt>" has a low precedence.
 So for example, "<tt>(yes|no)</tt>" is a way to match either "yes" or "no".
 </ol>