Make the et feature request link properly add the github label (#51594)

The label has a space in it, so it doesn't get properly added with the current link.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

tools/engine_tool/README.md

diff --git a/tools/engine_tool/README.md b/tools/engine_tool/README.md
index d3e1f42..0ab53e0 100644
--- a/tools/engine_tool/README.md
+++ b/tools/engine_tool/README.md
@@ -7,7 +7,7 @@
 
 > [!NOTE]
 > This tool is under development and is not yet ready for general use. Consider
-> filing a [feature request](https://github.com/flutter/flutter/issues/new?labels=e:engine-tool,team-engine).
+> filing a [feature request](https://github.com/flutter/flutter/issues/new?labels=e:%20engine-tool,team-engine).
 
 ## Prerequisites