Render `<button>` in `notes#new` template

This commit resolves the previous error:

Error:
UserSharesAMessageTest#test_visiting_the_index:
Capybara::ElementNotFound: Unable to find link or button "Share"
    test/system/user_shares_a_message_test.rb:10:in `block in <class:UserSharesAMessageTest>'

By declaring a [button] element with the text the click_on helper expects, this change addresses the previous test failure and advances our test out of the Exercise Phase and into the Verify Phase.

Our test’s assertion is raising a new error:

Failure:
UserSharesAMessageTest#test_visiting_the_index [/Users/seanpdoyle/src/prix-fixe/test/system/user_shares_a_message_test.rb:12]:
expected to find text "Hello, World!" in "Message Share"


      app/views/notes/new.html.erb

diff --git a/app/views/notes/new.html.erb b/app/views/notes/new.html.erb
index 855f00f..c2bbe44 100644
--- a/app/views/notes/new.html.erb
+++ b/app/views/notes/new.html.erb
@@ -1,2 +1,4 @@
 <label for="content">Message</label>
 <textarea id="content"></textarea>
+
+<button>Share</button>

Render <button> in notes#new template

Render `<button>` in `notes#new` template