fix: typo (AtsushiSakai#1023)

Author: RyuYamamoto

PathPlanning/DStarLite/d_star_lite.py

@@ -2,7 +2,7 @@
 D* Lite grid planning
 author: vss2sn ([email protected])
 Link to papers:
-D* Lite (Link: http://idm-lab.org/bib/abstracts/papers/aaai02b.pd)
+D* Lite (Link: http://idm-lab.org/bib/abstracts/papers/aaai02b.pdf)
 Improved Fast Replanning for Robot Navigation in Unknown Terrain
 (Link: http://www.cs.cmu.edu/~maxim/files/dlite_icra02.pdf)
 Implemented maintaining similarity with the pseudocode for understanding.