Update part0b.md

In the section on callback methods, uneven punctuation is used. Both should either be dashes or commas.

Author: rickdavids

src/content/0/en/part0b.md

@@ -274,7 +274,7 @@ xhttp.onreadystatechange = function() {
 }
 ```
 
-The mechanism of invoking event handlers is very common in JavaScript. Event handler functions are called [callback](https://developer.mozilla.org/en-US/docs/Glossary/Callback_function) functions. The application code does not invoke the functions itself, but the runtime environment - the browser, invokes the function at an appropriate time when the <i>event</i> has occurred.
+The mechanism of invoking event handlers is very common in JavaScript. Event handler functions are called [callback](https://developer.mozilla.org/en-US/docs/Glossary/Callback_function) functions. The application code does not invoke the functions itself, but the runtime environment - the browser - invokes the function at an appropriate time when the <i>event</i> has occurred.
 
 ### Document Object Model or DOM