Typo: dummpy.img -> dummy.img (#94)

Not sure if this was a typo or a snarky insider joke by a Perl/Ruby programmer....

Author: hartzell

pages/docs/user-docs/user-guide.md

@@ -113,7 +113,7 @@ Singularity does its best to abstract the complications of running an applicatio
 Some usage examples of Singularity can be seen as follows:
 
 ```bash
-$ singularity exec dummpy.img xterm
+$ singularity exec dummy.img xterm
 $ singularity exec dummy.img python script.py
 $ singularity exec dummy.img python < /path/to/python/script.py
 $ cat /path/to/python/script.py | singularity exec dummy.img python