Fixed conflicting accesskey, b=74984, r=akkana, sr=kin

editor/ui/composer/locale/en-US/editorOverlay.dtd

@@ -48,7 +48,7 @@
 <!ENTITY saveAsCmd.label "Save As...">
 <!ENTITY filesaveas.accesskey "a">
 <!ENTITY exportToTextCmd.label "Export to Text...">
-<!ENTITY fileexporttotext.accesskey "x">
+<!ENTITY fileexporttotext.accesskey "t">
 <!ENTITY saveAsCharsetCmd.label "Save As Charset...">
 <!ENTITY filesaveascharset.accesskey "h">
 <!ENTITY previewCmd.label "Browse Page">