chore(rustc_expand): fix typo in comment

Author: notriddle

compiler/rustc_expand/src/mbe/quoted.rs

@@ -72,7 +72,7 @@ pub(super) fn parse(
                                             // this with just `span.edition()`. A
                                             // `SyntaxContext::root()` from the current crate will
                                             // have the edition of the current crate, and a
-                                            // `SyntaxxContext::root()` from a foreign crate will
+                                            // `SyntaxContext::root()` from a foreign crate will
                                             // have the edition of that crate (which we manually
                                             // retrieve via the `edition` parameter).
                                             if span.ctxt() == SyntaxContext::root() {