[DOC] Inaccurate caption in Pagerank.ipynb

[charlesbluca]

Report incorrect documentation

Location of incorrect documentation

• https://github.com/rapidsai/cugraph/blob/branch-22.04/notebooks/link_analysis/Pagerank.ipynb

Describe the problems or issues found in the documentation
When highlighting the top vertices in the graph by out_degree, we actually print the top 4:

Suggested fix for documentation
Small fix - should just call head(3) instead of head(4)