Replace multiple trailing newlines with a single trailing newline

Sorry I didn’t get this in the last PR (#28864), I hadn’t thought of it.

Author: Carlos Liam

src/doc/nomicon/ownership.md

@@ -64,4 +64,3 @@ does in fact live as long as we needed. However it was *changed* while we had
 a reference into it. This is why Rust requires any references to freeze the
 referent and its owners.
 
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src/doc/trpl/ownership.md

@@ -236,13 +236,3 @@ complicated.
 Luckily, Rust offers a feature, borrowing, which helps us solve this problem.
 It’s the topic of the next section!
 
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src/libstd/os/freebsd/raw.rs

@@ -138,4 +138,3 @@ mod arch {
     }
 }
 
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src/test/compile-fail/transmute-imut-to-mut.rs

@@ -17,4 +17,3 @@ fn main() {
     //~^ ERROR mutating transmuted &mut T from &T may cause undefined behavior
 }
 
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