suppress warning: ambiguous first argument; put parentheses or a space even after `-' operator

Introduced at r58905
http://rubyci.s3.amazonaws.com/ubuntu/ruby-trunk/log/20170526T153003Z.diff.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58948 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/erb.rb

@@ -589,7 +589,7 @@ class ERB
     end
 
     def add_put_cmd(out, content)
-      out.push("#{@put_cmd} -#{content_dump(content)}")
+      out.push("#{@put_cmd}(-#{content_dump(content)})")
     end
 
     def add_insert_cmd(out, content)