Fix help: previous_release is mandatory

src/rebar.erl

@@ -211,7 +211,7 @@ delete-deps                          Delete fetched dependencies
 
 generate    [dump_spec=0/1]          Build release with reltool
 
-generate-upgrade  [previous_release=path]  Build an upgrade package
+generate-upgrade  previous_release=path  Build an upgrade package
 
 eunit       [suite=foo]              Run eunit [test/foo_tests.erl] tests
 ct          [suite=] [case=]         Run common_test suites in ./test