man: Silence mgba.6 mandoc warning.

  man -Tlint mgba

  man: mgba.6.gz:247:5: WARNING: cross reference to self: Xr mgba 6

  cross reference to self
    (mdoc) An Xr macro refers to a name and section matching the section
    of the present manual page and a name mentioned in an Nm macro in
    the NAME or SYNOPSIS section, or in an Fn or Fo macro in the
    SYNOPSIS. Consider using Nm or Fn instead of Xr.

  https://man.openbsd.org/mandoc.1

doc/mgba.6

@@ -244,7 +244,7 @@ is used.
 .Bl -tag -width Ds -compact
 .It Pa $XDG_CONFIG_HOME/mgba/config.ini
 Default
-.Xr mgba 6
+.Nm mgba
 configuration file.
 .It Pa portable.ini
 If this file exists in the current directory,