fix typo

Co-authored-by: jyn <[email protected]>

Author: RalfJung

Diff for: src/bootstrap/src/lib.rs

@@ -683,7 +683,7 @@ impl Build {
 
         if !self.config.dry_run() {
             {
-                // We first to a dry-run. This is a sanity-check to ensure that
+                // We first do a dry-run. This is a sanity-check to ensure that
                 // steps don't do anything expensive in the dry-run.
                 self.config.dry_run = DryRun::SelfCheck;
                 let builder = builder::Builder::new(self);