Fix escape behavior in single quote string literal

Remove addtion unnecessary \ after escape character \.

Author: aycabta

lib/rdoc/ruby_lex.rb

@@ -1337,7 +1337,7 @@ def identify_string(ltype, quoted = ltype, type = nil)
             when @ltype
               str << ch
             else
-              ungetc
+              str << ch
             end
           else
             str << read_escape