Revert default for OffsetHour modifier

The values was mistakenly changed and was originally `false`. This
change brings behavior in line with parsing the component.

time/src/format_description/modifier.rs

@@ -392,10 +392,10 @@ impl_const_default! {
     /// Creates a modifier that indicates the stringified value contains [one or more
     /// digits](SubsecondDigits::OneOrMore).
     @pub Subsecond => Self { digits: SubsecondDigits::OneOrMore };
-    /// Creates a modifier that indicates the value uses the `+` sign for all positive values
-    /// and is [padded with zeroes](Padding::Zero).
+    /// Creates a modifier that indicates the value only uses a sign for negative values and is
+    /// [padded with zeroes](Padding::Zero).
     @pub OffsetHour => Self {
-        sign_is_mandatory: true,
+        sign_is_mandatory: false,
         padding: Padding::Zero,
     };
     /// Creates a modifier that indicates the value is [padded with zeroes](Padding::Zero).

time/src/serde/mod.rs

@@ -403,7 +403,11 @@ impl<'a> Deserialize<'a> for Time {
 /// The format used when serializing and deserializing a human-readable `UtcOffset`.
 #[cfg(feature = "parsing")]
 const UTC_OFFSET_FORMAT: &[FormatItem<'_>] = &[
-    FormatItem::Component(Component::OffsetHour(modifier::OffsetHour::default())),
+    FormatItem::Component(Component::OffsetHour({
+        let mut m = modifier::OffsetHour::default();
+        m.sign_is_mandatory = true;
+        m
+    })),
     FormatItem::Optional(&FormatItem::Compound(&[
         FormatItem::Literal(b":"),
         FormatItem::Component(Component::OffsetMinute(modifier::OffsetMinute::default())),