Fix spelling errors in README.md

[notmessenger]

• "proxiy" at end of https://github.com/softlayer/sl-ember-store#using-adapters section
• "it's" should not have apostrophe near beginning of https://github.com/softlayer/sl-ember-store#using-adapters section
• "importan" near middle of the https://github.com/softlayer/sl-ember-store#options-parameter-in-the-storefind-method section, before the key examples