Update broken links in reference documentation.

Original Pull Request: #4267

src/main/asciidoc/preface.adoc

@@ -31,7 +31,7 @@ NoSQL stores have taken the storage world by storm. It is a vast domain with a p
 The starting point for learning about MongoDB is https://www.mongodb.org/[www.mongodb.org]. Here is a list of other useful resources:
 
 * The https://docs.mongodb.org/manual/[manual] introduces MongoDB and contains links to getting started guides, reference documentation, and tutorials.
-* The https://try.mongodb.org/[online shell] provides a convenient way to interact with a MongoDB instance in combination with the online https://docs.mongodb.org/manual/tutorial/getting-started/[tutorial.]
+* Visit https://learn.mongodb.com/[MongoDB University] for free training material and online courses.
 * MongoDB https://docs.mongodb.org/ecosystem/drivers/java/[Java Language Center].
 * Several https://www.mongodb.org/books[books] you can purchase.
 * Karl Seguin's online book: https://openmymind.net/mongodb.pdf[The Little MongoDB Book].

src/main/asciidoc/reference/mongodb.adoc

@@ -20,7 +20,7 @@ For most tasks, you should use `MongoTemplate` or the Repository support, which
 [[mongodb-getting-started]]
 == Getting Started
 
-An easy way to bootstrap setting up a working environment is to create a Spring-based project in https://spring.io/tools/sts[STS].
+An easy way to bootstrap setting up a working environment is to create a Spring-based project in https://spring.io/tools/[STS].
 
 First, you need to set up a running MongoDB server. Refer to the https://docs.mongodb.org/manual/core/introduction/[MongoDB Quick Start guide] for an explanation on how to startup a MongoDB instance. Once installed, starting MongoDB is typically a matter of running the following command: `${MONGO_HOME}/bin/mongod`