Fix method name in Random Sampling guide

[timvermeulen]

Replaces usage of randomElements with randomSample in the Random Sampling guide.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

Thanks, @timvermeulen!