Spelling (#703)

diff --git a/doxybuild.py b/doxybuild.py
index f18c9c0..862c1f4 100644
--- a/doxybuild.py
+++ b/doxybuild.py
@@ -156,7 +156,7 @@
 def main():
     usage = """%prog
     Generates doxygen documentation in build/doxygen.
-    Optionaly makes a tarball of the documentation to dist/.
+    Optionally makes a tarball of the documentation to dist/.
 
     Must be started in the project top directory.    
     """