Update source/reference/glossary.txt

Changed exit to exist in description for collection reference.

Author: pbradaric

source/reference/glossary.txt

@@ -64,7 +64,7 @@ Glossary
       set of fields, but typically all documents in a collection have
       a similar or related purpose for an application.
 
-      All collections exit within a single :term:`database`. The
+      All collections exist within a single :term:`database`. The
       namespace within a database for collections are flat.
 
       See :ref:`faq-dev-namespace` and :doc:`/core/document` for more