rails: change rg alias to rgen (#7858)

The rg alias conflicted with ripgrep.

plugins/rails/rails.plugin.zsh

@@ -43,7 +43,7 @@ alias rc='rails console'
 alias rcs='rails console --sandbox'
 alias rd='rails destroy'
 alias rdb='rails dbconsole'
-alias rg='rails generate'
+alias rgen='rails generate'
 alias rgm='rails generate migration'
 alias rp='rails plugin'
 alias ru='rails runner'