Update for upcoming markdown changes. (rust-lang#1880)

Author: ehuss

src/doc/rustc-dev-guide/src/borrow_check/drop_check.md

@@ -152,3 +152,5 @@ One possible way these inconsistencies can be fixed is by MIR building
 to be more pessimistic, probably by making `Ty::needs_drop` weaker, or
 alternatively, changing `dropck_outlives` to be more precise, requiring
 fewer regions to be live.
+
+[^core]: This is the core assumption of [#110288](https://github.com/rust-lang/rust/issues/110288) and [RFC 3417](https://github.com/rust-lang/rfcs/pull/3417).

src/doc/rustc-dev-guide/src/building/bootstrapping.md

@@ -429,7 +429,7 @@ Building rustdoc for stage1 (x86_64-unknown-linux-gnu)
 These steps use the provided (downloaded, usually) compiler to compile the
 local Rust source into libraries we can use.
 
-### Copying stage0 {std,rustc}
+### Copying stage0 \{std,rustc\}
 
 This copies the library and compiler artifacts from Cargo into
 `stage0-sysroot/lib/rustlib/{target-triple}/lib`

src/doc/rustc-dev-guide/src/llvm-coverage-instrumentation.md

@@ -247,10 +247,9 @@ properly-configured variables in LLVM IR, according to very specific
 details of the [_LLVM Coverage Mapping Format_][coverage-mapping-format]
 (Version 6).[^llvm-and-covmap-versions]
 
-[^llvm-and-covmap-versions]:
-The Rust compiler (as of <!-- date-check: --> Jul 2023) supports _LLVM Coverage Mapping Format_ 6.
-The Rust compiler will automatically use the most up-to-date coverage mapping format
-version that is compatible with the compiler's built-in version of LLVM.
+[^llvm-and-covmap-versions]: The Rust compiler (as of <!-- date-check: --> Jul 2023) supports _LLVM Coverage Mapping Format_ 6.
+    The Rust compiler will automatically use the most up-to-date coverage mapping format
+    version that is compatible with the compiler's built-in version of LLVM.
 
 ```rust
 pub fn finalize<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>) {