fix unclosed div in MergeUI

[RayBB]

When trying to build vue in development mode it was failing with an error (see below).

Turns out we had an unclosed div that the IDE was complaining about too.
The production mode was swallowing the error and browsers were probably making a lucky guess of where to end the div.

Contributes to #7547

Technical

Error I was getting

npx vue-cli-service build --no-clean --watch --mode development --dest static/build/components/prod
uction --target wc --name ol-MergeUI openlibrary/components/MergeUI.vue

⠋  Building for development as web component...

 ERROR  Failed to compile with 1 error                                                                                          9:32:00 AM

 error  in ./openlibrary/components/MergeUI.vue?vue&type=template&id=468bf6df&shadow

Module Error (from ./node_modules/@vue/vue-loader-v15/lib/loaders/templateLoader.js):
(Emitted value instead of an instance of Error) 

  Errors compiling template:

  tag <div> has no matching end tag.

  1  |  
     |   
  2  |  <div id="app">
     |  ^^^^^^^^^^^^^^
  3  |    <MergeTable :olids="olids" :show_diffs="show_diffs" :primary="primary" ref="mergeTable"/>

Testing

Check that diffs option still works fine (it does)

Screenshot

screenshot

Stakeholders

@jimchamp

[codecov-commenter]

Codecov Report

Merging #8385 (0f7f431) into master (35d6b72) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #8385   +/-   ##
=======================================
  Coverage   16.66%   16.66%           
=======================================
  Files          83       83           
  Lines        4422     4422           
  Branches      757      757           
=======================================
  Hits          737      737           
  Misses       3202     3202           
  Partials      483      483           

[cdrini]

Threw on testing looks good!