MAINT: fix typo that printed wrong spin information in gw/source.py

Closes #723 (closed)

From the current master branch, the typo was only present in two lines, this commit fixes that.

AUTHORS.md

@@ -97,3 +97,4 @@ Marc Penuliar
 Andrew Fowlie
 Martin White
 Peter Tsun-Ho Pang
+Alexandre Sebastien Goettel