Fixed typo
diff --git a/include/polarssl/x509_crt.h b/include/polarssl/x509_crt.h
index e3443d0..93340ec 100644
--- a/include/polarssl/x509_crt.h
+++ b/include/polarssl/x509_crt.h
@@ -253,7 +253,7 @@
* certificate to perform an RSA key exchange).
*
* \return 0 is these uses of the certificate are allowed,
- * POLARSSL_ERR_X509_BAD_INPUT_DATA if the keyUsage extenson
+ * POLARSSL_ERR_X509_BAD_INPUT_DATA if the keyUsage extension
* is present but does not contain all the bits set in the
* usage argument.
*