ObjCXX: Fix detection of C++20 default compiler mode

Fix a typo from commit 9f3c70a333 (CUDA, CXX, OBJCXX: C++23 support with
Clang 12, 2020-12-04, v3.20.0-rc1~321^2).

Modules/CMakeOBJCXXCompilerId.mm.in

@@ -32,7 +32,7 @@ char const* qnxnto = "INFO" ":" "qnxnto[]";
 const char* info_language_standard_default = "INFO" ":" "standard_default["
 #if CXX_STD > 202002L
   "23"
-#elfif CXX_STD > 201703L
+#elif CXX_STD > 201703L
   "20"
 #elif CXX_STD >= 201703L
   "17"