[Chapel Merge] Fix a `make docs` typo introduced in #19584 that M

Branch: refs/heads/main
Revision: 15f2258
Author: bradcray
Link: Fix a `make docs` typo introduced in #19584 that Michael caught post-merge by bradcray · Pull Request #19763 · chapel-lang/chapel · GitHub
Log Message:

Merge pull request #19763 from bradcray/fix-chpl-home-makefile-var

Fix a make docs typo introduced in #19584 that Michael caught post-merge

[reviewed by @mppf]

Whatever I was doing in testing wasn't sensitive to this, possibly
because I was operating in an environment that had CHPL_HOME set
in the shell. Thanks for noticing that, @mppf!

Modified Files:
M modules/Makefile

Compare: https://github.com/chapel-lang/chapel/compare/589cfb2b01d4...15f2258e87c2