mod_brotli: Fix a minor typo in the description of BrotliAlterETag

that has been referring to httpd 2.2.x.

There's no mod_brotli in 2.2.x.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1790853 13f79535-47bb-0310-9956-ffa450edef68