fix(Make): eliminate discrepancy between OSX and Linux builds

Turns out that OSX `cp -R` does something subtly different from
Linux `cp -R`, and it's based on the presence of the trailing slash
in the source of the copy.

Happily, it looks like we can fix it by just getting rid of that slash.
Therefore this commit.

Author: coriolinus

Makefile

@@ -732,7 +732,7 @@ docs-kotlin-deps := $(JVM_LIB) $(jvm-deps) $(KOTLIN_SOURCES)
 $(STAMPS)/docs-kotlin: $(docs-kotlin-deps)
 	cd crypto-ffi/bindings && ./gradlew jvm:dokkaGeneratePublicationHtml
 	mkdir -p target/kotlin/doc
-	cp -R crypto-ffi/bindings/jvm/build/dokka/html/ target/kotlin/doc
+	cp -R crypto-ffi/bindings/jvm/build/dokka/html target/kotlin/doc
 	$(TOUCH_STAMP)
 
 .PHONY: docs-kotlin