Nanoc: Mention nanoc.yaml rather than config.yaml

Nanoc.gitignore

 # For projects using Nanoc (http://nanoc.ws/)
 
-# Default location for output (needs to match output_dir's value found in config.yaml)
+# Default location for output (needs to match output_dir's value found in nanoc.yaml)
 output/
 
 # Temporary file directory