fix typo in docs

[tanmaykm]

fix minor typo in docs of merge method

[codecov]

Codecov Report

Merging #295 (d7ec7a6) into main (e7a908d) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #295      +/-   ##
==========================================
- Coverage   87.72%   87.71%   -0.01%     
==========================================
  Files          15       15              
  Lines        1751     1750       -1     
==========================================
- Hits         1536     1535       -1     
  Misses        215      215              

Impacted Files	Coverage Δ
src/join.jl	94.00% <ø> (ø)
src/columns.jl	90.20% <0.00%> (-0.04%)	⬇️

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