fix typo

Author: douyw

README.md

@@ -72,7 +72,7 @@ Usage:
   rails generate sqlpp11gen:tab NAME [OUTPUT_PATH] [FILENAME_PREFIX] [FILENAME_EXT] [options]
 ```
 
-- The `NAME` is neccessary, it's the table name you want to generate code from.
+- The `NAME` is necessary, it's the table name you want to generate code from.
 - `OUTPUT_PATH` allows you to specify the output path for the generated code. The default path is 'app/cxxes'.
 - `FILENAME_PREFIX` allows you to append a filename prefix. The default one is 'tab_'.
 - `FILENAME_EXT` allows you to choose the filename ext. The default one is 'hpp'.