write: Update page extension and heading

pages/linux/write.md

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+# write
+
+> Write a message on the terminal of a specified logged in user. (ctrl-C to stop writing messages)
+> To find out all terminal_ids of all users active on the system see the 'who' command
+
+- Send a message to user {{username}} on terminal {{terminal_id}}:
+
+`write {{username}} {{terminal_id}}`
+
+- send message to user 'testuser' on terminal /dev/tty/5
+
+`write {{testuser}} {{tty/5}}`
+
+- send message to user 'jhondoe' on pseudo terminal /dev/pts/5
+
+`write {{jhondoe}} {{pts/5}}`