Remove unnecessary `:`

Before:

`.../irb/init.rb:280: warning: :LoadError: cannot load such file -- hoge`

After:

`.../irb/init.rb:280: warning: LoadError: cannot load such file -- hoge`

[ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61175 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/irb/init.rb

@@ -277,7 +277,7 @@ module IRB # :nodoc:
       begin
         require m
       rescue LoadError => err
-        warn ":#{err.class}: #{err}", uplevel: 0
+        warn "#{err.class}: #{err}", uplevel: 0
       end
     end
   end