src/xz/xz.1: Correct misused two-fonts macros

Output is from: test-groff -b -e -mandoc -T utf8 -rF0 -t -w w -z

  [ "test-groff" is a developmental version of "groff" ]

Input file is ./src/xz/xz.1

<src/xz/xz.1>:408 (macro BR): only 1 argument, but more are expected
<src/xz/xz.1>:1009 (macro BR): only 1 argument, but more are expected
<src/xz/xz.1>:1743 (macro BR): only 1 argument, but more are expected
<src/xz/xz.1>:1920 (macro BR): only 1 argument, but more are expected
<src/xz/xz.1>:2213 (macro BR): only 1 argument, but more are expected

  Output from nroff and troff is unchanged, except for a font change of a
full stop (.).

Signed-off-by: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>