tools: fix docopen target

Adds to prerequisites the output file to open. Previously, this would
open an out-of-date version of the file, or fail if it hasn't been built
before.

PR-URL: #35062
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>

Makefile

@@ -767,8 +767,8 @@ out/doc/api/all.json: $(apidocs_json) tools/doc/alljson.js | out/doc/api
 	$(call available-node, tools/doc/alljson.js)
 
 .PHONY: docopen
-docopen: $(apidocs_html)
-	@$(PYTHON) -mwebbrowser file://$(PWD)/out/doc/api/all.html
+docopen: out/doc/api/all.html
+	@$(PYTHON) -mwebbrowser file://$(abspath $<)
 
 .PHONY: docserve
 docserve: $(apidocs_html) $(apiassets)