[book][validation] Fixing typo - closes #465

Author: weaverryan

book/validation.rst

@@ -428,7 +428,7 @@ options can be specified in this way.
 This is purely meant to make the configuration of the most common option of
 a constraint shorter and quicker.
 
-IFf you'r ever unsure of how to specify an option, either check the API documentation
+If you're ever unsure of how to specify an option, either check the API documentation
 for the constraint or play it safe by always passing in an array of options
 (the first method shown above).