Fix @backtace typo, add spec

lib/pry/pry_instance.rb

@@ -89,7 +89,7 @@ class Pry
     @indent        = Pry::Indent.new
     @command_state = {}
     @eval_string   = ""
-    @backtace      = options[:backtrace] || caller
+    @backtrace     = options[:backtrace] || caller
 
     refresh_config(options)
     push_initial_binding(options)

spec/pry_spec.rb

@@ -428,4 +428,14 @@ describe Pry do
       }
     end
   end
+
+  describe "a fresh instance" do
+    it "should use `caller` as its backtrace" do
+      location  = "#{__FILE__}:#{__LINE__ + 1}"
+      backtrace = Pry.new.backtrace
+
+      backtrace.should.not.be.nil
+      backtrace.any? { |l| l.include?(location) }.should.be.true
+    end
+  end
 end