gfguarante: update citation

* spot/twaalgos/gfguarantee.hh: Properly cite the LICS'18 paper.
* doc/spot.bib: Add the entry.

doc/spot.bib

@@ -1,4 +1,3 @@
-
 @InProceedings{	  babiak.12.tacas,
   author	= {Tom{\'a}{\v{s}} Babiak and Mojm{\'i}r
 		  K{\v{r}}et{\'i}nsk{\'y} and Vojt{\v{e}}ch {\v{R}}eh{\'a}k
@@ -457,6 +456,20 @@
   doi		= {10.1007/978-3-642-01702-5_17}
 }
 
+@InProceedings{	  esparza.18.lics,
+  author	= {Javier Esparza and Jan K\v{r}et{\'{\i}}nsk{\'{y}} and
+		  Salomon Sickert},
+  title		= {One Theorem to Rule Them All: {A} Unified Translation of
+		  {LTL} into $\omega$-Automata},
+  booktitle	= {Proceedings of the 33rd Annual {ACM/IEEE} Symposium on
+		  Logic in Computer Science (LICS'18)},
+  pages		= {384--393},
+  year		= {2018},
+  editor	= {Anuj Dawar and Erich Gr{\"{a}}del},
+  publisher	= {ACM},
+  doi		= {10.1145/3209108.3209161}
+}
+
 @InProceedings{	  etessami.00.concur,
   author	= {Kousha Etessami and Gerard J. Holzmann},
   title		= {Optimizing {B\"u}chi Automata},