Add missing @Flag initializer in example (apple#657)

The documentation as presented doesn't compile. An initializer is
required for a boolean flag.

rdar://132579907

Co-authored-by: Christian Goetze <[email protected]>

Sources/ArgumentParser/Parsable Properties/OptionGroup.swift

@@ -17,7 +17,7 @@
 /// ```swift
 /// struct GlobalOptions: ParsableArguments {
 ///     @Flag(name: .shortAndLong)
-///     var verbose: Bool
+///     var verbose: Bool = false
 ///
 ///     @Argument var values: [Int]
 /// }