Change the nginx app version to a set tag

Want to avoid moving tags like using 'stable'. Therefore,
specify the specifc nginx version/tag.

Update from comment review:
- https://github.com/helm/helm/pull/5662#discussion_r280122531

Signed-off-by: Martin Hickey <martin.hickey@ie.ibm.com>

pkg/chartutil/create.go

@@ -72,9 +72,8 @@ type: application
 version: 0.1.0
 
 # This is the version number of the application being deployed. This version number should be
-# incremented each time you make changes to the application. Depending on the application,
+# incremented each time you make changes to the application.
-# 'stable' can signify the latest stable version of the application.
+appVersion: 1.16.0
-appVersion: stable
 `
 
 const defaultValues = `# Default values for %s.