Fix typos in docs

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -1.50 :warning:

Comparison is base (486b984) 49.25% compared to head (fbe1c21) 47.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #41 +/- ## ========================================== - Coverage 49.25% 47.76% -1.50% ========================================== Files 6 6 Lines 134 134 ========================================== - Hits 66 64 -2 - Misses 68 70 +2 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/TuringLang/TuringCallbacks.jl/pull/41/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TuringLang)

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[yebai]

Thanks @pitmonticone!