DOCS-14610 correct sentences

Author: ianf-mongodb

source/tutorial/update-documents-with-aggregation-pipeline.txt

@@ -67,9 +67,9 @@ Example 1
    Specifically, the pipeline consists of a :pipeline:`$set` stage
    which adds the ``test3`` field (and sets its value to ``98``) to the
    document and sets the ``modified`` field to the current datetime.
-   For the current datetime, the operation uses the aggregation
-   variable :variable:`NOW` for the (to access the variable, prefix with ``$$``
-   and enclose in quotes).
+   The operation uses the aggregation variable :variable:`NOW` for the 
+   current datetime. To access the variable, prefix with  ``$$`` and enclose 
+   in quotes.
 
    To verify the update, you can query the collection:
 
@@ -125,18 +125,18 @@ Example 2
 
    Specifically, the pipeline consists of:
 
-   - a :pipeline:`$replaceRoot` stage with an
+   - a :pipeline:`$replaceRoot` stage with a
      :expression:`$mergeObjects` expression to set default values for
      the ``quiz1``, ``quiz2``, ``test1`` and ``test2`` fields. The
      aggregation variable :variable:`ROOT` refers to the current
-     document being modified (to access the variable, prefix with
-     ``$$`` and enclose in quotes). The current document fields will
+     document being modified. To access the variable, prefix with
+     ``$$`` and enclose in quotes. The current document fields will
      override the default values.
 
    - a :pipeline:`$set` stage to update the ``modified`` field to the
-     current datetime. For the current datetime, the operation uses the
-     aggregation variable :variable:`NOW` for the (to access the
-     variable, prefix with ``$$`` and enclose in quotes).
+     current datetime. The operation uses the aggregation variable 
+     :variable:`NOW` for the current datetime. To access the variable, 
+     prefix with ``$$`` and enclose in quotes.
 
    To verify the update, you can query the collection:
 
@@ -201,11 +201,11 @@ Example 3
    - a :pipeline:`$set` stage to calculate the truncated average value
      of the ``tests`` array elements and to update the ``modified``
      field to the current datetime. To calculate the truncated average,
-     the stage uses the :expression:`$avg` and :expression:`$trunc`
-     expressions. For the current datetime, the operation uses the
-     aggregation variable :variable:`NOW` for the (to access the
-     variable, prefix with ``$$`` and enclose in quotes)
-   
+     the stage uses the :group:`$avg` and :expression:`$trunc`
+     expressions. The operation uses the aggregation variable 
+     :variable:`NOW` for the current datetime. To access the variable,
+     prefix with ``$$`` and enclose in quotes.
+
    - a :pipeline:`$set` stage to add the ``grade`` field based on the
      ``average`` using the :expression:`$switch` expression.