Fixed a typo in docs

Replaced `chart` command by `helm`.
Signed-off-by: Sergey Kozlov <mail.sergey.kozlov@gmail.com>

docs/chart_template_guide/notes_files.md

 # Creating a NOTES.txt File
 
-In this section we are going to look at Helm's tool for providing instructions to your chart users. At the end of a `chart install` or `chart upgrade`, Helm can print out a block of helpful information for users. This information is highly customizable using templates.
+In this section we are going to look at Helm's tool for providing instructions to your chart users. At the end of a `helm install` or `helm upgrade`, Helm can print out a block of helpful information for users. This information is highly customizable using templates.
 
 To add installation notes to your chart, simply create a `templates/NOTES.txt` file. This file is plain text, but it is processed like as a template, and has all the normal template functions and objects available.