u-a(8): Fix prerm invocation arguments

It is usually called in prerm's remove and deconfigure, not install.

Closes: #761025

Reported-by: jre <jre.winesim@gmail.com>

debian/changelog

@@ -34,6 +34,8 @@ dpkg (1.17.14) UNRELEASED; urgency=low
     - Add a mention of packaging helpers support in dpkg-maintscript-helper(1).
       Closes: #759754
     - Move dpkg-buildflags(1) generic feature area details to the section top.
+    - Fix prerm invocation arguments in update-alternatives(8).
+      Reported by jre <jre.winesim@gmail.com>. Closes: #761025
   * Add new deps_iterate() function to Dpkg::Deps. Closes: #759747
     Based on a patch by Dima Kogan <dima@secretsauce.net>.
   * Add support for relative symlinks to dpkg-maintscript-helper symlink_to_dir

man/update-alternatives.8

@@ -85,7 +85,7 @@ is usually called from the
 .B postinst
 (configure) or
 .B prerm
-(install) scripts in Debian packages.
+(remove and deconfigure) scripts in Debian packages.
 .PP
 It is often useful for a number of alternatives to be synchronized,
 so that they are changed as a group; for example, when several versions