Rollup merge of rust-lang#45058 - hunteke:fix_rustc_private_typo_2017Oct, r=shepmaster

Fix typo, per rust-lang#45057.

This looks like a simple string -- one character -- fix.  Given that I'm currently running low on battery, I have not actually compiled and tested this.  But I am fully confident this passes muster.  If not, I'll be maintainer-educated, yes?  ;-)

Author: kennytm

src/librustc/middle/stability.rs

@@ -429,7 +429,7 @@ impl<'a, 'tcx> Index<'tcx> {
             // while maintaining the invariant that all sysroot crates are unstable
             // by default and are unable to be used.
             if tcx.sess.opts.debugging_opts.force_unstable_if_unmarked {
-                let reason = "this crate is being loaded from the sysroot, and \
+                let reason = "this crate is being loaded from the sysroot, an \
                               unstable location; did you mean to load this crate \
                               from crates.io via `Cargo.toml` instead?";
                 let stability = tcx.intern_stability(Stability {