Adjust section titles in the user guide

I found `package-managers` ambiguous since it could mean
installing `rustup` using a package manager,
or installing `rustup` while already using externally-managed
`rust` toolchains.

Hopefully this change will make that ambiguity disappear.

Author: rami3l

doc/user-guide/book.toml

@@ -6,3 +6,6 @@ title = "The rustup book"
 edit-url-template = "https://github.com/rust-lang/rustup/edit/master/doc/user-guide/{path}"
 git-repository-url = "https://github.com/rust-lang/rustup/tree/master/doc/user-guide"
 site-url = "https://rust-lang.github.io/rustup/"
+
+[output.html.redirect]
+"/installation/package-managers.html" = "already-installed-rust.html"

doc/user-guide/src/SUMMARY.md

@@ -3,9 +3,9 @@
 [Introduction](index.md)
 - [Installation](installation/index.md)
     - [Windows](installation/windows.md)
-        - [MSVC Prerequistes](installation/windows-msvc.md)
+        - [MSVC prerequisites](installation/windows-msvc.md)
     - [Other installation methods](installation/other.md)
-    - [Package managers](installation/package-managers.md)
+    - [Already installed Rust?](installation/already-installed-rust.md)
 - [Concepts](concepts/index.md)
     - [Channels](concepts/channels.md)
     - [Toolchains](concepts/toolchains.md)

…ide/src/installation/package-managers.md …c/installation/already-installed-rust.mddoc/user-guide/src/installation/package-managers.md renamed to doc/user-guide/src/installation/already-installed-rust.md doc/user-guide/src/installation/package-managers.md renamed to doc/user-guide/src/installation/already-installed-rust.md

@@ -1,4 +1,4 @@
-# Package managers
+# Already installed Rust?
 
 Several Linux distributions package Rust, and you may wish to use the packaged
 toolchain, such as for distribution package development. You may also wish to