Update app.html

Was receiving following error when using latest Meteor version with Blade templating engine: "Started proxy.                             
=> Errors prevented startup:                  
   
   While building the application:
   app/app.html:48: Unexpected closing template tag
   ...t;</button>   {{/if}}   </form>
   ^
   
=> Your application has errors. Waiting for file change.
=> Started MongoDB."

Moved form element inside template block.