Fix single quote in double quotes in R Markdown options

[mwouts]

Closes #1079

[codecov]

Codecov Report

Merging #1081 (150f9ac) into main (64b84b7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 150f9ac differs from pull request most recent head 1d45bc9. Consider uploading reports for the commit 1d45bc9 to get more accurate results

@@           Coverage Diff           @@
##             main    #1081   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files         120      120           
  Lines       11120    11134   +14     
=======================================
+ Hits        11014    11028   +14     
  Misses        106      106           

Impacted Files	Coverage Δ
jupytext/cell_metadata.py	98.99% <100.00%> (ø)
jupytext/formats.py	98.46% <100.00%> (+<0.01%)	⬆️
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_read_simple_markdown.py	100.00% <100.00%> (ø)
tests/test_read_simple_rmd.py	100.00% <100.00%> (ø)

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