add missing period

Author: krishahn

versions/3.0.md

@@ -1729,7 +1729,7 @@ The key value used to identify the callback object is an expression, evaluated a
 ##### Patterned Fields
 Field Pattern | Type | Description
 ---|:---:|---
-<a name="callbackExpression"></a>{expression} | [Path Item Object](#pathItemObject) | A Path Item Object used to define a callback request and expected responses
+<a name="callbackExpression"></a>{expression} | [Path Item Object](#pathItemObject) | A Path Item Object used to define a callback request and expected responses.
 
 This object can be extended with [Specification Extensions](#specificationExtensions).