Clarify section title (#5226)

As stated by @schollii in https://github.com/helm/helm/issues/4505#issuecomment-415886732

 "Overriding Values from a Parent Chart" is unclear. This changes that text to "Overriding Values of a Child Chart". See @scholli's comment for justification.

Signed-off-by: Nahuel Lascano <nlascano@eryx.co>