docs: enforce "treesitter" spelling #27110

It's the "tree-sitter" project, but "treesitter" in our code and docs.

src/nvim/buffer_defs.h

@@ -713,7 +713,7 @@ struct file_buffer {
 
   // Measurements of the deleted or replaced region since the last update
   // event. Some consumers of buffer changes need to know the byte size (like
-  // tree-sitter) or the corresponding UTF-32/UTF-16 size (like LSP) of the
+  // treesitter) or the corresponding UTF-32/UTF-16 size (like LSP) of the
   // deleted text.
   size_t deleted_bytes;
   size_t deleted_bytes2;