Fix incorrect RFC3339 offset parsing

[zhu-he]

Fix a bug when parsing RFC3339 offset in range from -00:01 to -00:59.

[codecov]

Codecov Report

Merging #478 (11fe2fb) into main (9a3f985) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #478   +/-   ##
=======================================
  Coverage   99.48%   99.48%           
=======================================
  Files          70       70           
  Lines        7201     7206    +5     
=======================================
+ Hits         7164     7169    +5     
  Misses         37       37           

Impacted Files	Coverage Δ
src/parsing/parsable.rs	98.75% <100.00%> (+0.01%)	⬆️

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[jhpratt]

Good catch. Thank you!