Change patch-version help text to be more clear

[#169603689](https://www.pivotaltracker.com/story/show/169603689)
 As a platform operator I can understand the purpose of the \patch-version" flag more clearly as a result of the help text"

Co-authored-by: Daniel Basner <dbasner@pivotal.io>

commandparser/package_stemcell.go

@@ -112,7 +112,7 @@ func (p *PackageCmd) SetFlags(f *flag.FlagSet) {
 
 	f.StringVar(&p.outputConfig.OutputDir, "outputDir", "", "Output directory, default is the current working directory.")
 	f.StringVar(&p.outputConfig.OutputDir, "o", "", "Output directory (shorthand)")
-	f.StringVar(&patchVersion, "patch-version", "", "String appended at end of stemcell version as a patch version. (Optional)")
+	f.StringVar(&patchVersion, "patch-version", "", "Number or name of the patch version for the stemcell being built (e.g: for 2019.12.3 the string would be \"3\")")
 }
 
 func (p *PackageCmd) Execute(_ context.Context, f *flag.FlagSet, _ ...interface{}) subcommands.ExitStatus {