Fix the CAGRA paper citation (#1788)

This PR fixes the citation info for the CAGRA paper in README.md.
I forgot to write closing '```' in the citation part in README.md

rel: #1787

Authors:
  - tsuki (https://github.com/enp1s0)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #1788

README.md

@@ -393,6 +393,7 @@ If citing the single-linkage agglomerative clustering APIs, please consider the
       archivePrefix={arXiv},
       primaryClass={cs.LG}
 }
+```
 
 If citing CAGRA, please consider the following bibtex:
 ```bibtex
@@ -404,3 +405,4 @@ If citing CAGRA, please consider the following bibtex:
       archivePrefix={arXiv},
       primaryClass={cs.DS}
 }
+```