* doc/tl/tl.bib: Properly quote PSL.

doc/tl/tl.bib

-
 @InProceedings{	  babiak.12.tacas,
   author	= {Thom{\'a}{\v{s}} Babiak and Mojm{\'i}r
 		  K{\v{r}}et{\'i}nsk{\'y} and Vojt{\v{e}}ch {\v{R}e}eh{\'a}k
@@ -73,7 +72,7 @@
 		  Circuits and Systems},
   number	= 10,
   pages		= {1737--1750},
-  title		= {Symbolic Compilation of PSL},
+  title		= {Symbolic Compilation of {PSL}},
   volume	= 27,
   year		= 2008,
   date		= {2009-03-20},