build: fixup typo regarding branch name

Fix branch name path indication via '+' where it's missing a slash.

Change-Id: I826647e4da6af1202ae956eaa894f07b180ccb0a
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index db229b4..e0104db 100644
--- a/configure.ac
+++ b/configure.ac
@@ -74,7 +74,7 @@
                     [HAVE_UAPI_LINUX_LPC_CTRL_H=""],
                     [HAVE_UAPI_LINUX_LPC_CTRL_H="-I linux/aspeed-lpc-ctrl.h"])
     AS_IF([test "$HAVE_UAPI_LINUX_LPC_CTRL_H" != ""],
-        AC_MSG_WARN([Could not find linux/aspeed-lpc-ctrl.h: Attempting to download locally for building from openbmc/linux/+dev-4.18])
+        AC_MSG_WARN([Could not find linux/aspeed-lpc-ctrl.h: Attempting to download locally for building from openbmc/linux/+/dev-4.18])
         AC_SUBST([BT_BMC_DL],
                  [`mkdir -p linux;wget https://raw.githubusercontent.com/openbmc/linux/dev-4.18/include/uapi/linux/aspeed-lpc-ctrl.h -O linux/aspeed-lpc-ctrl.h`])
     )