disable spellcheck, autocorrect and autocapitalize on the filter input

the text entered here are likely to be functions and technical terms and as such spellcheck, autocapitalize and autocorrect would better be disabled.

Author: coliff

src/default_theme/index._

@@ -21,6 +21,9 @@
             placeholder='Filter'
             id='filter-input'
             class='col12 block input'
+            spellcheck='false'
+            autocapitalize='off'
+            autocorrect='off'
             type='text' />
           <div id='toc'>
             <ul class='list-reset h5 py1-ul'>