Fix: Documentation of --only in tests cmd

This pull request fixes the documentation for the —only flag of the
tests cmd.

Library/Homebrew/dev-cmd/tests.rb

-#:  * `tests` [`-v`] [`--coverage`] [`--generic`] [`--no-compat`] [`--only=`<test_script/test_method>] [`--seed` <seed>] [`--trace`] [`--online`] [`--official-cmd-taps`]:
+#:  * `tests` [`-v`] [`--coverage`] [`--generic`] [`--no-compat`] [`--only=`<test_script:test_method>] [`--seed` <seed>] [`--trace`] [`--online`] [`--official-cmd-taps`]:
 #:    Run Homebrew's unit and integration tests.
 
 require "fileutils"

completions/zsh/_brew

@@ -678,7 +678,7 @@ _brew_test_bot() {
 }
 
 # brew tests [-v] [--coverage] [--generic] [--no-compat]
-#   [--only=test_script/test_method] [--seed seed] [--trace] [--online]
+#   [--only=test_script:test_method] [--seed seed] [--trace] [--online]
 #   [--official-cmd-taps]:
 _brew_tests() {
   _arguments \