Fix broken link in fragment-shaders.md (#12667)

_Description of what this PR is changing or adding, and why:_ This PR
changes the link to _Writing efficient shaders_ that is found under
https://docs.flutter.dev/ui/design/graphics/fragment-shaders#performance-considerations.
It seems that after relocating the engine docs
(flutter/flutter@e068a3e),
this link didn't change accordingly.

_Issues fixed by this PR (if any):_
#12665

_PRs or commits this PR depends on (if any):_

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Author: iakdis

src/content/ui/design/graphics/fragment-shaders.md

@@ -366,7 +366,7 @@ this is more efficient than creating a new
 For a more detailed guide on writing performant shaders,
 check out [Writing efficient shaders][] on GitHub.
 
-[Writing efficient shaders]: {{site.repo.flutter}}/blob/main/engine/src/flutter/impeller/docs/shader_optimization.md
+[Writing efficient shaders]: {{site.repo.flutter}}/blob/main/docs/engine/impeller/docs/shader_optimization.md
 
 ### Other resources