man: Fix an example by quoting a filter expression

man: Fix an example by quoting a filter expression

It's to prevent the shell from (mis-)interpreting the square brackets.

e.g. if there is a file named tcp1 in the current directory,
with tcp[13] == 2, the executed command will be:
tcpdump -i xl0 tcp1 == 2
Result: tcpdump: can't parse filter expression: syntax error

(cherry picked from commit 5db1dbf206cadf0d063334fa849746f74e42c588)

[skip ci]