Fixed small typo in demo code (#9425)

imgui_demo.cpp

diff --git a/imgui_demo.cpp b/imgui_demo.cpp
index 673a024..e0bbd9a 100644
--- a/imgui_demo.cpp
+++ b/imgui_demo.cpp
@@ -4121,7 +4121,7 @@
         ImGui::BeginDisabled();
         ImGui::Button("Disabled item", sz);
         if (ImGui::IsItemHovered(ImGuiHoveredFlags_ForTooltip))
-            ImGui::SetTooltip("I am a a tooltip for a disabled item.");
+            ImGui::SetTooltip("I am a tooltip for a disabled item.");
         ImGui::EndDisabled();
 
         ImGui::TreePop();