bin/systemd-analyze-plot: do not use read without a variable

shellcheck 0.10.0 reports:

    In bin/systemd-analyze-plot line 32:
        read -r 2> /dev/null
        ^--^ SC3061 (warning): In POSIX sh, read without a variable is undefined.

Silence this by using an unused variable.
koalaman/shellcheck#1134 suggests using `$_`
for such a use-case.

bin/systemd-analyze-plot

@@ -29,7 +29,7 @@ if systemd-analyze plot > "$TEMP"
 then
     xdg-open "$TEMP"
     echo "Press 'Enter' to quit."
-    read -r 2> /dev/null
+    read -r _ 2> /dev/null
 else
     exit $?
 fi