Actualize link to spec chapter about status codes

[andrew-demb]

The previous link redirects to (probably moved) the repository - https://github.com/graphql/graphql-over-http

And the new repository doesn't have an anchor #status-codes for the main repository page.

[codecov-commenter]

Codecov Report

Merging #463 (e8541a8) into master (4824b96) will increase coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #463      +/-   ##
============================================
+ Coverage     96.25%   96.36%   +0.11%     
- Complexity     1625     1631       +6     
============================================
  Files           146      146              
  Lines          4085     4100      +15     
============================================
+ Hits           3932     3951      +19     
+ Misses          153      149       -4     

Impacted Files	Coverage Δ
src/Http/HttpCodeDecider.php	100.00% <ø> (ø)
src/TypeRegistry.php	100.00% <0.00%> (ø)
src/Types/TypeAnnotatedInterfaceType.php	100.00% <0.00%> (ø)
src/Mappers/AbstractTypeMapper.php	99.39% <0.00%> (+0.01%)	⬆️
src/AnnotationReader.php	94.94% <0.00%> (+0.02%)	⬆️
src/Mappers/RecursiveTypeMapper.php	94.92% <0.00%> (+0.10%)	⬆️
src/Mappers/Parameters/TypeHandler.php	98.06% <0.00%> (+0.69%)	⬆️
src/Annotations/Input.php	95.23% <0.00%> (+16.29%)	⬆️

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[oojacoboo]

Thanks @andrew-demb