fix(isAlpha) th-TH locale missing

[ipiranhaa]

I found the th-TH locale option miss from the isAlpha document but already implemented.

Just add th-Th locale option on README.md

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1757 (fff9abe) into master (4ec30b7) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1757   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          102       102           
  Lines         2029      2029           
  Branches       457       457           
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  Hits          2029      2029           

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[tux-tn]

Thank you for your PR @ipiranhaa
Looks like the locale is missing from isAlpha documentation as well, can you add it there too?