manpage: irb command take `--examples` options

Closes Homebrew/homebrew#40985.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

share/man/man1/brew.1

@@ -224,10 +224,10 @@ To install a newer version of HEAD use \fBbrew rm <foo> && brew install \-\-HEAD
 If \fB\-\-git\fR is passed, Homebrew will create a Git repository, useful for creating patches to the software\.
 .
 .IP "\(bu" 4
-\fBirb [\-\-example]\fR: Enter the interactive Homebrew Ruby shell\.
+\fBirb [\-\-examples]\fR: Enter the interactive Homebrew Ruby shell\.
 .
 .IP
-If \fB\-\-example\fR is passed, several examples will be shown\.
+If \fB\-\-examples\fR is passed, several examples will be shown\.
 .
 .IP "\(bu" 4
 \fBleaves\fR: Show installed formulae that are not dependencies of another installed formula\.