Update write-operations-introduction.txt

Author: shlok

source/core/write-operations-introduction.txt

@@ -47,7 +47,7 @@ The following diagram shows the same query in SQL:
 
 .. example::
 
-   The following operation inserts a new documents into the ``users``
+   The following operation inserts a new document into the ``users``
    collection. The new document has four fields ``name``, ``age``, and
    ``status``, and an ``_id`` field. MongoDB always adds the ``_id``
    field to the new document if that field does not exist.