Fix grammar in part4b.md

Author: aleichtm

src/content/4/en/part4b.md

@@ -153,7 +153,7 @@ Checking the value of the header uses a bit strange looking syntax:
 .expect('Content-Type', /application\/json/)
 ```
 
-The desired value is now defined as [regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions) or in short regex. The regex starts and ends with a slash /, because the desired string <i>application/json</i> also contains the same slash, it is preceded by a \ so that it is not interpreted as a regex termination character.
+The desired value is now defined as [regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions) or in short regex. The regex starts and ends with a slash /, and because the desired string <i>application/json</i> also contains the same slash, it is preceded by a \ so that it is not interpreted as a regex termination character.
 
 In principle, the test could also have been defined as a string