changing README links from master to main

[parmeet]

changing all links from master to main

[codecov]

Codecov Report

Merging #1392 (c6776ce) into main (b40eb2c) will decrease coverage by 2.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1392      +/-   ##
==========================================
- Coverage   79.29%   77.12%   -2.17%     
==========================================
  Files          68       68              
  Lines        3279     3279              
==========================================
- Hits         2600     2529      -71     
- Misses        679      750      +71     

Impacted Files	Coverage Δ
torchtext/vocab/vectors.py	50.90% <0.00%> (-38.79%)	⬇️
torchtext/utils.py	86.50% <0.00%> (-5.56%)	⬇️

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