Change tabs to spaces in mojo README

TBR=sky@chromium.org

Bug: 714018
Change-Id: I684311c0b3f4b213f59508c3f5cee528598fb89a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948519Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721013}

mojo/public/cpp/system/README.md

@@ -244,7 +244,7 @@ class PipeReader {
     // after SimpleWatcher destruction.
     watcher_.Watch(pipe_.get(), MOJO_HANDLE_SIGNAL_READABLE,
                    base::BindRepeating(&PipeReader::OnReadable,
-				                       base::Unretained(this)));
+                                       base::Unretained(this)));
   }
 
   ~PipeReader() {}