Fix doc charts indent (#4940)

Signed-off-by: Jintao Zhang <zhangjintao9020@gmail.com>
Signed-off-by: Kevin Labesse <kevin@labesse.me>

docs/charts.md

@@ -259,7 +259,7 @@ Tags - The tags field is a YAML list of labels to associate with this chart.
 In the top parent's values, all charts with tags can be enabled or disabled by
 specifying the tag and a boolean value.
 
-````
+```yaml
 # parentchart/requirements.yaml
 dependencies:
   - name: subchart1
@@ -278,8 +278,8 @@ dependencies:
       - back-end
           - subchart2
 
-````
-````
+```
+```yaml
 # parentchart/values.yaml
 
 subchart1: