Fixed typo in reference documentation

Author: jyemin

docs/reference/content/upgrading.md

@@ -13,7 +13,7 @@ The 4.2 release is a minor release as per the definition of [semantic versioning
 changes in this release for users upgrading from the 4.1 release.
 
 Note that in this release the Reactive Streams driver has taken an implementation dependency on 
-[Project Reactor](https://projectreactor. io/).  The minimum supported Project Reactor version is *Californium-SR23*.
+[Project Reactor](https://projectreactor.io/).  The minimum supported Project Reactor version is *Californium-SR23*.
 
 # Upgrading to the 4.1 Driver