Small typo fix usefull => useful

Author: wbt

README.md

@@ -112,7 +112,7 @@ limitHandler | <ul><li><code>false</code>&nbsp;**(default)**</li><li><code>funct
 useTempFiles | <ul><li><code>false</code>&nbsp;**(default)**</li><li><code>true</code></ul> | By default this module uploads files into RAM. Setting this option to True turns on using temporary files instead of utilising RAM. This avoids memory overflow issues when uploading large files or in case of uploading lots of files at same time.
 tempFileDir | <ul><li><code>String</code>&nbsp;**(path)**</li></ul> | Path to store temporary files.<br />Used along with the <code>useTempFiles</code> option. By default this module uses 'tmp' folder in the current working directory.<br />You can use trailing slash, but it is not necessary.
 parseNested | <ul><li><code>false</code>&nbsp;**(default)**</li><li><code>true</code></li></ul> | By default, req.body and req.files are flattened like this: <code>{'name': 'John', 'hobbies[0]': 'Cinema', 'hobbies[1]': 'Bike'}</code><br /><br/>When this option is enabled they are parsed in order to be nested like this: <code>{'name': 'John', 'hobbies': ['Cinema', 'Bike']}</code>
-debug | <ul><li><code>false</code>&nbsp;**(default)**</li><li><code>true</code></ul> | Turn on/off upload process logging. Can be usefull for troubleshooting.
+debug | <ul><li><code>false</code>&nbsp;**(default)**</li><li><code>true</code></ul> | Turn on/off upload process logging. Can be useful for troubleshooting.
 
 # Help Wanted
 Looking for additional maintainers. Please contact `richardgirges [ at ] gmail.com` if you're interested. Pull Requests are welcomed!