build: small cleanup in error message in make.bat

Contrary to bash, double quotes cannot be used to group
arguments in Windows shell, so they were being printed as
literals by the echo command.

Since a literal '>' is present in the string, it is sufficient
to escape it correctly through '^'.

Change-Id: Icc8c92b3dc8d813825adadbe3d921a38d44a1a94
Reviewed-on: https://go-review.googlesource.com/97056Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/make.bat

@@ -121,7 +121,7 @@ goto end
 
 :bootstrapfail
 echo ERROR: Cannot find %GOROOT_BOOTSTRAP%\bin\go.exe
-echo "Set GOROOT_BOOTSTRAP to a working Go tree >= Go 1.4."
+echo Set GOROOT_BOOTSTRAP to a working Go tree ^>= Go 1.4.
 
 :fail
 set GOBUILDFAIL=1