Use more general word to avoid getting stale

Co-authored-by: Tshepang Mbambo <[email protected]>

Author: smanilov

src/about-this-guide.md

@@ -3,7 +3,7 @@
 This guide is meant to help document how rustc – the Rust compiler – works,
 as well as to help new contributors get involved in rustc development.
 
-There are nine parts to this guide:
+There are several parts to this guide:
 
 1. [Building and debugging `rustc`][p1]:
    Contains information that should be useful no matter how you are contributing,