Fix typo in an OID name

fixes #314
diff --git a/library/oid.c b/library/oid.c
index 1bd47d7..c71cd9b 100644
--- a/library/oid.c
+++ b/library/oid.c
@@ -267,7 +267,7 @@
         MBEDTLS_X509_EXT_KEY_USAGE,
     },
     {
-        { ADD_LEN( MBEDTLS_OID_EXTENDED_KEY_USAGE ),   "id-ce-keyUsage",           "Extended Key Usage" },
+        { ADD_LEN( MBEDTLS_OID_EXTENDED_KEY_USAGE ),   "id-ce-extKeyUsage",        "Extended Key Usage" },
         MBEDTLS_X509_EXT_EXTENDED_KEY_USAGE,
     },
     {