typo and formatting

fixed a typo and modified some line formatting justification while I'm here :)

compiler/rustc_expand/src/placeholders.rs

@@ -313,25 +313,24 @@ impl<'a, 'b> MutVisitor for PlaceholderExpander<'a, 'b> {
             // Implement the proposal described in
             // https://github.com/rust-lang/rust/issues/61733#issuecomment-509626449
             //
-            // The macro invocation expands to the list of statements.
-            // If the list of statements is empty, then 'parse'
-            // the trailing semicolon on the original invocation
-            // as an empty statement. That is:
+            // The macro invocation expands to the list of statements. If the
+            // list of statements is empty, then 'parse' the trailing semicolon
+            // on the original invocation as an empty statement. That is:
             //
             // `empty();` is parsed as a single `StmtKind::Empty`
             //
-            // If the list of statements is non-empty, see if the
-            // final statement alreayd has a trailing semicolon.
+            // If the list of statements is non-empty, see if the final
+            // statement already has a trailing semicolon.
             //
-            // If it doesn't have a semicolon, then 'parse' the trailing semicolon
-            // from the invocation as part of the final statement,
+            // If it doesn't have a semicolon, then 'parse' the trailing
+            // semicolon from the invocation as part of the final statement,
             // using `stmt.add_trailing_semicolon()`
             //
             // If it does have a semicolon, then 'parse' the trailing semicolon
             // from the invocation as a new StmtKind::Empty
 
-            // FIXME: We will need to preserve the original
-            // semicolon token and span as part of #15701
+            // FIXME: We will need to preserve the original semicolon token and
+            // span as part of #15701
             let empty_stmt = ast::Stmt {
                 id: ast::DUMMY_NODE_ID,
                 kind: ast::StmtKind::Empty,