common.mk: help of exam [ci skip]

* common.mk (help): mention `exam` target.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53053 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -1084,6 +1084,7 @@ help: PHONY
 	"  gdb:             runs test.rb by miniruby under gdb" \
 	"  gdb-ruby:        runs test.rb by ruby under gdb" \
 	"  check:           equals make test test-all" \
+	"  exam:            equals make check test-rubyspec" \
 	"  test:            ruby core tests" \
 	"  test-all:        all ruby tests [TESTOPTS=-j4 TESTS=\"<test files>\"]" \
 	"  test-rubyspec:   run RubySpec test suite" \