Replace deprecated egrep with grep in make docs-linkcheckbroken

Author: stevepiercy

Makefile

@@ -170,7 +170,7 @@ linkcheck: deps  ## Run linkcheck
 
 .PHONY: linkcheckbroken
 linkcheckbroken: deps  ## Run linkcheck and show only broken links
-	cd $(DOCS_DIR) && $(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck | GREP_COLORS='0;31' egrep -wi broken --color=auto
+	cd $(DOCS_DIR) && $(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck | GREP_COLORS='0;31' grep -wi broken --color=auto
 	@echo
 	@echo "Link check complete; look for any errors in the above output " \
 		"or in $(BUILDDIR)/linkcheck/ ."