Fixed docker tag file name in README.md

Author: AlexSkrypnyk

Diff for: README.md

@@ -39,7 +39,7 @@ The functionality of building PHP extensions is provided by [mlocati's docker-ph
 Using the docker tag from subsequent scripts
 --------------------------------------------
 
-Once the Docker image is built and tagged, the name of the tag will be written to a file called `docker-tag` within the current working directory. This can then be read by a subsequent script.
+Once the Docker image is built and tagged, the name of the tag will be written to a file called `docker_tag` within the current working directory. This can then be read by a subsequent script.
 
 Version caching php-build
 -------------------------
@@ -58,4 +58,4 @@ If you found this repository helpful, please consider [sponsoring the developer]
 [mlocati]: https://github.com/mlocati/docker-php-extension-installer
 [mlocati-readme]: https://github.com/mlocati/docker-php-extension-installer#supported-php-extensions
 [issues]: https://github.com/php-actions/php-build/issues
-[sponsor]: https://github.com/sponsors/g105b
+[sponsor]: https://github.com/sponsors/g105b