Add missing "If" in ProblemDetails section.

Author: msporny

index.html

@@ -4757,9 +4757,9 @@ <h3>Problem Details</h3>
           </dd>
           <dt>code</dt>
           <dd>
-The `code` [=property=] is OPTIONAL.
-present, its value MUST be an integer that identifies the type of the problem.
-Integer codes are useful in systems that only provide integer return values.
+The `code` [=property=] is OPTIONAL. If present, its value MUST be an integer
+that identifies the type of the problem. Integer codes are useful in systems
+that only provide integer return values.
           </dd>
           <dt>title</dt>
           <dd>