Replace a broken YouTube link (#1295)

src/mir/dataflow.md

@@ -17,8 +17,10 @@ some basic terminology, such as "transfer function", "fixpoint" and "lattice".
 If you're unfamiliar with these terms, or if you want a quick refresher,
 [*Static Program Analysis*] by Anders Møller and Michael I. Schwartzbach is an
 excellent, freely available textbook. For those who prefer audiovisual
-learning, the Goethe University Frankfurt has published a series of short
-[lectures on YouTube][goethe] in English that are very approachable.
+learning, we previously recommended a series of short lectures
+by the Goethe University Frankfurt on YouTube, but it has been deleted for some reasons.
+See [this PR][pr-1295] for the context and [this comment][pr-1295-comment]
+for the alternative lectures.
 
 ## Defining a Dataflow Analysis
 
@@ -229,6 +231,7 @@ the example below:
 [`apply_call_return_effect`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.Analysis.html#tymethod.apply_call_return_effect
 [`into_engine`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.Analysis.html#method.into_engine
 [`lattice`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/lattice/index.html
-[goethe]: https://www.youtube.com/watch?v=NVBQSR_HdL0&list=PL_sGR8T76Y58l3Gck3ZwIIHLWEmXrOLV_&index=2
+[pr-1295]: https://github.com/rust-lang/rustc-dev-guide/pull/1295
+[pr-1295-comment]: https://github.com/rust-lang/rustc-dev-guide/pull/1295#issuecomment-1118131294
 [lattice]: https://en.wikipedia.org/wiki/Lattice_(order)
 [wiki]: https://en.wikipedia.org/wiki/Data-flow_analysis#Basic_principles