Update google/auth/crypt/rsa.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: daniel-sanche

google/auth/crypt/rsa.py

@@ -116,7 +116,7 @@ def sign(self, message):
 
     @classmethod
     def from_string(cls, key, key_id=None):
-        """Construct an Signer instance from a private key in PEM format.
+        """Construct a Signer instance from a private key in PEM format.
 
         Args:
             key (str): Private key in PEM format.