Fix docstring for gamma_inc.rgamma1pm1

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.37%. Comparing base (b27f0ad) to head (f627118).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #469 +/- ## ========================================== - Coverage 94.44% 92.37% -2.07% ========================================== Files 14 14 Lines 2914 2978 +64 ========================================== - Hits 2752 2751 -1 - Misses 162 227 +65 ``` | [Flag](https://app.codecov.io/gh/JuliaMath/SpecialFunctions.jl/pull/469/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/JuliaMath/SpecialFunctions.jl/pull/469/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath) | `92.37% <ø> (-2.07%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#carryforward-flags-in-the-pull-request-comment) to find out more.

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