src/script/xzgrep.1: Remove superfluous '.RB'

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/scripts/xzgrep.1

<src/scripts/xzgrep.1>:20 (macro RB): only 1 argument, but more are expected
<src/scripts/xzgrep.1>:23 (macro RB): only 1 argument, but more are expected
<src/scripts/xzgrep.1>:26 (macro RB): only 1 argument, but more are expected
<src/scripts/xzgrep.1>:29 (macro RB): only 1 argument, but more are expected
<src/scripts/xzgrep.1>:32 (macro RB): only 1 argument, but more are expected

 "abc..." does not mean the same as "abc ...".

  The output from nroff and troff is unchanged except for the space
between "file" and "...".

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