Update flash message on Passwords#update.

Issues
------
- Closes #45

Author: stevepolitodesign

app/controllers/passwords_controller.rb

@@ -35,7 +35,7 @@ def update
       elsif @user.password_reset_token_has_expired?
         redirect_to new_password_path, alert: "Incorrect email or password."
       elsif @user.update(password_params)
-        redirect_to login_path, notice: "Signed in."
+        redirect_to login_path, notice: "Password updated."
       else
         flash.now[:alert] = @user.errors.full_messages.to_sentence
         render :edit