Fix typo in jwt docs. (#332)

diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index bea70ad..d63c50b 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -200,7 +200,7 @@
     Args:
         token (str): The encoded JWT.
         certs (Union[str, bytes, Mapping[str, Union[str, bytes]]]): The
-            certificate used to validate the JWT signatyre. If bytes or string,
+            certificate used to validate the JWT signature. If bytes or string,
             it must the the public key certificate in PEM format. If a mapping,
             it must be a mapping of key IDs to public key certificates in PEM
             format. The mapping must contain the same key ID that's specified