unstable-book: fix capitalization
diff --git a/src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md b/src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md
index 6895f23..1520b86 100644
--- a/src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md
+++ b/src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md
@@ -23,7 +23,7 @@
In particular, nightly is built with beta, and beta is built with stable.
Since the standard library and compiler both use unstable features, `RUSTC_BOOTSTRAP` is required so that we can use the previous version to build them.
-## Why is this environment variable so easy to use for people not in the rust project?
+## Why is this environment variable so easy to use for people not in the Rust project?
Originally, `RUSTC_BOOTSTRAP` required passing in a hash of the previous compiler version, to discourage using it for any purpose other than bootstrapping.
That constraint was later relaxed; see <https://github.com/rust-lang/rust/issues/36548> for the discussion that happened at that time.
