fix rst url syntax error in the citation page
@@ -5,4 +5,4 @@ Citing and acknowledging Lightkurve
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If you find Lightkurve useful in your research and would like to acknowledge it,
-please check our entry on `zenodo https://doi.org/10.5281/zenodo.1181928`.
+please check our entry on `zenodo https://doi.org/10.5281/zenodo.1181928`_.
