Fixed a typo.

index.js

@@ -71,7 +71,7 @@ async function main() {
     const current_version = await increaseVersion(url_pattern, start_version);
     const export_to = core.getInput("export-to");
     core.info(`The new version determend is ${current_version}`);
-    exportVariable("current_version", `${current_version}`);
+    core.exportVariable("current_version", `${current_version}`);
     core.setOutput("current_version", `${current_version}`);
   } catch (error) {
     core.setFailed(`Action failed with error: ${error}`);