guides/chroot.md: emphasize the need to select a repository.

From comments on IRC, it seems people accidentally skip this part and
get XBPS errors because it can't find the repodata for the XBPS_ARCH in
use.

Suggested by cvm on IRC, written by me.

Author: ericonr

src/installation/guides/chroot.md

@@ -81,10 +81,10 @@ Follow only one of the two following subsections.
 
 ### The XBPS Method
 
-Select a [mirror](../../xbps/repositories/mirrors/index.md) and use the
-[appropriate URL](../../xbps/repositories/index.md#the-main-repository) for the
+Select a [mirror](../../xbps/repositories/mirrors/index.md) and **use the
+[appropriate URL](../../xbps/repositories/index.md#the-main-repository)** for the
 type of system you wish to install. For simplicity, save this URL to a shell
-variable for later use, e.g.:
+variable. A glibc installation, for example, would use:
 
 ```
 # REPO=https://alpha.de.repo.voidlinux.org/current