Update dependency_scanning_scans descriptions

ee/config/metrics/counts_28d/20210216183828_dependency_scanning_scans.yml

 ---
 data_category: operational
 key_path: usage_activity_by_stage_monthly.secure.dependency_scanning_scans
-description: Number of Dependency Scanning security scans run
+description: Monthly count of Dependency Scanning security scans run
 product_section: sec
 product_stage: secure
 product_group: group::composition analysis

ee/config/metrics/counts_all/20210216175220_dependency_scanning_scans.yml

 ---
 data_category: operational
 key_path: usage_activity_by_stage.secure.dependency_scanning_scans
-description: Total number of users running Dependency Scanning Scans
+description: Total count of Dependency Scanning security scans run
 product_section: sec
 product_stage: secure
 product_group: group::composition analysis