Fix typo, improve wording

Author: matthurne

source/applications/aggregation.txt

@@ -201,7 +201,7 @@ to scan only the matching documents in a collection.
 .. without affecting the result by moving the :agg:pipeline:`$match`
 .. operator in front of the :agg:pipeline:`$project`.
 
-In future versions there may be pipleine optimization phase in the
+In future versions there may be an optimization phase in the
 pipleine that reorders the operations to increase performance without
 affecting the result. However, at this time place
 :agg:pipeline:`$match` operators at the beginning of the pipeline when