merge tools: fix typo in Beyond Compare's 3 command line

Fix a typo error preventing 'base' to be displayed properly.

contrib/mergetools.hgrc

@@ -51,7 +51,7 b' filemerge.executable=/Developer/Applicat'
 filemerge.args=-left $other -right $local -ancestor $base -merge $output
 filemerge.gui=True
 
-beyondcompare3.args=$local $other $base $output /ro /lefttitle=local /centerfile=base /righttitle=other /automerge /reviewconflicts /solo
+beyondcompare3.args=$local $other $base $output /ro /lefttitle=local /centertitle=base /righttitle=other /automerge /reviewconflicts /solo
 beyondcompare3.regkey=Software\Scooter Software\Beyond Compare 3
 beyondcompare3.regname=ExePath
 beyondcompare3.gui=True