Replace complete with merged in MR Analytics docs

After some discussion we decided that it makes sense to use the phrase
"merge requests merged" instead of "merge requests completed". This
updates the documentation to reflect.

doc/user/analytics/merge_request_analytics.md

@@ -15,7 +15,7 @@ Merge Request Analytics helps you understand the efficiency of your code review
 
 Merge Request Analytics displays information that will help you evaluate the efficiency and productivity of your merge request process.
 
-The Throughput chart shows the number of completed merge requests, by month. Merge request throughput is
+The Throughput chart shows the number of merge requests merged, by month. Merge request throughput is
 a common measure of productivity in software engineering. Although imperfect, the average throughput can
 be a meaningful benchmark of your team's overall productivity.
 
@@ -40,7 +40,7 @@ The following visualizations and data are available, representing all merge requ
 
 ### Throughput chart
 
-The throughput chart shows the number of completed merge requests per month.
+The throughput chart shows the number of merge requests merged per month.
 
 ![Throughput chart](img/mr_throughput_chart_v13_3.png "Merge Request Analytics - Throughput chart showing merge requests merged in the past 12 months")