Fix rdoc comment indent

These kind of comments seem to be often written as "=begin rdoc", which
wasn't accounted for.

Fixes issue #29

indent/ruby.vim

@@ -126,13 +126,13 @@ function s:PrevNonBlankNonString(lnum)
     " Go in and out of blocks comments as necessary.
     " If the line isn't empty (with opt. comment) or in a string, end search.
     let line = getline(lnum)
-    if line =~ '^=begin$'
+    if line =~ '^=begin'
       if in_block
 	let in_block = 0
       else
 	break
       endif
-    elseif !in_block && line =~ '^=end$'
+    elseif !in_block && line =~ '^=end'
       let in_block = 1
     elseif !in_block && line !~ '^\s*#.*$' && !(s:IsInStringOrComment(lnum, 1)
 	  \ && s:IsInStringOrComment(lnum, strlen(line)))