um: Fix typo in vector driver transport option definition

No big problem as "raw" and "gre" have the same length, but could go wrong if
they don't in the future.
Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_user.h

@@ -17,7 +17,7 @@
 #define TRANS_TAP_LEN strlen(TRANS_TAP)
 
 #define TRANS_GRE "gre"
-#define TRANS_GRE_LEN strlen(TRANS_RAW)
+#define TRANS_GRE_LEN strlen(TRANS_GRE)
 
 #define TRANS_L2TPV3 "l2tpv3"
 #define TRANS_L2TPV3_LEN strlen(TRANS_L2TPV3)