[5.1.x] Fixed #36162 -- Fixed the black Makefile docs rule to work on macOS.

The `make black` target in the docs directory used Linux-specific
syntax for its `find` command. Changed to syntax that also works
on macOS and other BSD Unix variants.
Backport of 248d845 from main.

Author: medmunds

docs/Makefile

@@ -169,7 +169,7 @@ spelling:
 
 black:
 	@mkdir -p $(BUILDDIR)/black
-	find -name "*.txt" -not -path "./_build/*" -not -path "./_theme/*" \
+	find . -name "*.txt" -not -path "./_build/*" -not -path "./_theme/*" \
 		| xargs blacken-docs --rst-literal-block; echo $$? > "$(BUILDDIR)/black/output.txt"
 	@echo
 	@echo "Code blocks reformatted"