Use "heading" consistently on the UI

novelwriter/gui/projtree.py

@@ -1836,7 +1836,7 @@ class _TreeContextMenu(QMenu):
 
     def _itemHeader(self) -> None:
         """Check if there is a header that can be used for rename."""
-        if hItem := SHARED.project.index.getItemHeader(self._handle, "T0001"):
+        if hItem := SHARED.project.index.getItemHeading(self._handle, "T0001"):
             action = self.addAction(self.tr("Rename to Heading"))
             action.triggered.connect(
                 lambda: self.projTree.renameTreeItem(self._handle, hItem.title)
@@ -1935,7 +1935,7 @@ class _TreeContextMenu(QMenu):
             action.triggered.connect(lambda: tree._mergeDocuments(tHandle, True))
 
         if isFile:
-            action = menu.addAction(self.tr("Split Document by Headers"))
+            action = menu.addAction(self.tr("Split Document by Headings"))
             action.triggered.connect(lambda: tree._splitDocument(tHandle))
 
         return