Fix some small grammar mistakes in documentation

[Kriskras99]

No description provided.

[codecov-commenter]

Codecov Report

Merging #613 (d6637c2) into master (5a536d0) will decrease coverage by 0.04%.
The diff coverage is 11.11%.

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@@            Coverage Diff             @@
##           master     #613      +/-   ##
==========================================
- Coverage   64.48%   64.44%   -0.04%     
==========================================
  Files          36       36              
  Lines       16921    16923       +2     
==========================================
- Hits        10911    10906       -5     
- Misses       6010     6017       +7     

Flag	Coverage Δ
unittests	64.44% <11.11%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/serde_helpers.rs	0.00% <ø> (ø)
src/de/mod.rs	68.98% <11.11%> (-0.30%)	⬇️

... and 2 files with indirect coverage changes

[Mingun]

Thanks for your fixes! I pretty sure that all of them are correct, but leave that to @dralley to merge it, just in case.