Align with style conventions (#1038)

For guidance, see https://grafana.com/docs/writers-toolkit/write/style-guide/capitalization-punctuation/#sentence-case-capitalization.

Author: osg-grafana

mixin-utils/utils.libsonnet

@@ -67,7 +67,7 @@ local g = import 'grafana-builder/grafana.libsonnet';
           },
           format: 'time_series',
           intervalFactor: 2,
-          legendFormat: '%(legend)s99th Percentile' % legend,
+          legendFormat: '%(legend)s99th percentile' % legend,
           refId: 'A',
           step: 10,
         },
@@ -81,7 +81,7 @@ local g = import 'grafana-builder/grafana.libsonnet';
           },
           format: 'time_series',
           intervalFactor: 2,
-          legendFormat: '%(legend)s50th Percentile' % legend,
+          legendFormat: '%(legend)s50th percentile' % legend,
           refId: 'B',
           step: 10,
         },