Fix parsing of group fields from text format

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.13 :tada:

Comparison is base (ac5af4f) 75.39% compared to head (7ed3bb7) 75.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #35 +/- ## ========================================== + Coverage 75.39% 75.52% +0.13% ========================================== Files 31 31 Lines 5226 5226 ========================================== + Hits 3940 3947 +7 + Misses 1286 1279 -7 ``` | [Impacted Files](https://codecov.io/gh/andrewhickman/prost-reflect/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Hickman) | Coverage Δ | | |---|---|---| | [prost-reflect/src/dynamic/text\_format/parse/mod.rs](https://codecov.io/gh/andrewhickman/prost-reflect/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Hickman#diff-cHJvc3QtcmVmbGVjdC9zcmMvZHluYW1pYy90ZXh0X2Zvcm1hdC9wYXJzZS9tb2QucnM=) | `81.45% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://codecov.io/gh/andrewhickman/prost-reflect/pull/35/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Hickman) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Hickman). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Hickman)

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