title: I can't remove • and some other special characters such as '- using tm_map
body: <p>I search through the questions and able to replace • in my first set of command.
  But when I apply to my corpus, it doesn't work, the • still appear.
  The corpus has 6570 elements,2.3mb, so it seems to be valid.</p>
  
  <pre><code>&gt; x &lt;- ". R Tutorial"
  &gt; gsub("•","",x)
  [1] ". R Tutorial"
  
  &gt; removeSpecialChars &lt;- function(x) gsub("•","",x)
  &gt; corpus2=tm_map(corpus2, removeSpecialChars)
  &gt; print(corpus2[[6299]][1])
  [1] "• R tutorial • success– october"
  &gt; ##remove special characters
  </code></pre>