Fix names of error in test descriptions of Kernel#raise

This was probably a leftover of a copy-paste, these should check
TypeError instead of ArgumentError.

Author: herwinw

core/kernel/raise_spec.rb

@@ -51,11 +51,11 @@
     -> { raise(cause: nil) }.should raise_error(ArgumentError, "only cause is given with no arguments")
   end
 
-  it "raises an ArgumentError when given cause is not an instance of Exception" do
+  it "raises a TypeError when given cause is not an instance of Exception" do
     -> { raise "message", cause: Object.new }.should raise_error(TypeError, "exception object expected")
   end
 
-  it "doesn't raise an ArgumentError when given cause is nil" do
+  it "doesn't raise a TypeError when given cause is nil" do
     -> { raise "message", cause: nil }.should raise_error(RuntimeError, "message")
   end