correct name of validate_arguments in docs

[samuelcolvin]

I had a brain fart while writing the docs for validate_arguments, @StephenBrown2 pointed this out in #1179 (comment).

Hopefully I've caught every one.

[codecov]

Codecov Report

Merging #1375 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1375   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3723      3736   +13     
  Branches       735       739    +4     
=========================================
+ Hits          3723      3736   +13     

Impacted Files	Coverage Δ
pydantic/mypy.py	100.00% <0.00%> (ø)
pydantic/types.py	100.00% <0.00%> (ø)
pydantic/utils.py	100.00% <0.00%> (ø)
pydantic/errors.py	100.00% <0.00%> (ø)
pydantic/fields.py	100.00% <0.00%> (ø)
pydantic/schema.py	100.00% <0.00%> (ø)
pydantic/validators.py	100.00% <0.00%> (ø)

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[StephenBrown2]

I was just about to submit a PR with these changes, including some punctuation I've noted.
Not sure about the emphasis of *pydantic* in only one place.

[samuelcolvin]

I tried to use *pydantic* everywhere, but I've slipped a bit.

I'm go though the rest tomorrow.