Update install_r_macos.md

[kbodwin]

Dollar sign in $PATH should not be escaped.

[llrs]

Nice addition, perhaps a better place would be the R Dev guide, the how to install R for MacOs is missing. Copying/porting this code there might help more users.

[aitap]

I think this is generated from install_r_macos.Rmd:

rcwg/working_documents/install_r_macos.Rmd

Lines 47 to 54 in a11014c

	```{sh}
	# Intel
	export PATH="/opt/R/x86_64/bin:\${PATH}"
	```
	```{sh}
	# Apple Silicon (M1, M2, ...)
	export PATH="/opt/R/arm64/bin:\${PATH}"
	```