Fix a small error in the `std` docstring

[sostock]

std has no means argument. The argument is always called mean, even if it contains several values.

Since the next paragraph already explains the use of mean together with dims, the wrong part can just be deleted.

[codecov-commenter]

Codecov Report

Base: 96.94% // Head: 96.94% // No change to project coverage :thumbsup:

Coverage data is based on head (a9c6920) compared to base (20fbe57). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #133 +/- ## ======================================= Coverage 96.94% 96.94% ======================================= Files 1 1 Lines 426 426 ======================================= Hits 413 413 Misses 13 13 ``` | [Impacted Files](https://codecov.io/gh/JuliaStats/Statistics.jl/pull/133?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats) | Coverage Δ | | |---|---|---| | [src/Statistics.jl](https://codecov.io/gh/JuliaStats/Statistics.jl/pull/133/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats#diff-c3JjL1N0YXRpc3RpY3Muamw=) | `96.94% <ø> (ø)` | | 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=JuliaStats). 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=JuliaStats)

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