Fixed typo. (ocornut#2108, ocornut#2083)

Author: ocornut

docs/README.md

@@ -25,7 +25,7 @@ Dear ImGui is self-contained within a few files that you can easily copy and com
 - imgui_widgets.cpp
 - imgui_internal.h
 - imconfig.h (empty by default, user-editable)
-- imstb_rect_pack.h
+- imstb_rectpack.h
 - imstb_textedit.h
 - imstb_truetype.h