tz: add one bug fix

I meant to include this in #234, but it slipped my notice.

Basically, if two offsets compare equal as-is, then it should always
return true. Before this, if our parsed offset was `-00:44:30` and the
time zone offset was `-00:44:30`, we were returning them as not equal
because we skipped to rounding the latter to the nearest minute.

I fixed this in the parser implementation but forgot to fix it in the
doc example. That's what this commit does.

src/tz/offset.rs

@@ -1836,7 +1836,7 @@ impl OffsetConflict {
     ///      for the given datetime unambiguously has offset -00:44:30",
     /// );
     /// let is_equal = |parsed: Offset, candidate: Offset| {
-    ///     candidate.round(Unit::Minute).map_or(
+    ///     parsed == candidate || candidate.round(Unit::Minute).map_or(
     ///         parsed == candidate,
     ///         |candidate| parsed == candidate,
     ///     )