Fix spelling error in description

Author: Scott Kitterman

setup.py

@@ -126,7 +126,7 @@ def run(self):
 if tuple(setuptools.__version__.split('.')) < ('36', '4', '0'):
     raise Exception('authheaders requires setuptools version 36.4.0 or later')
 
-DESC = """Python module for generating email authentication headers: Authheaders can generate both authentication results header fields and DKIM/ ARC sighatures. It can perform DKIM, SPF, and DMARC validation, and the results are packaged into a single Authentication-Results header. It can also DKIM and ARC sign messages and output the corresponding signature header fields. """
+DESC = """Python module for generating email authentication headers: Authheaders can generate both authentication results header fields and DKIM/ ARC signatures. It can perform DKIM, SPF, and DMARC validation, and the results are packaged into a single Authentication-Results header. It can also DKIM and ARC sign messages and output the corresponding signature header fields. """
 
 setup(
     name = "authheaders",