Update user-guide.md

Author: k2heart

docs/user-guide.md

@@ -131,7 +131,7 @@ $ fosslight_reuse report -f src/FOSSLight-Report.xlsx
 ```
 
 ## 📁 Result
- an output file name is specified with -o, a result file is created with that name.
+If an output file name is specified with -o, a result file is created with that name.
 - FOSSLight-Report_[datetime].xlsx : When the oss-pkg-info.yaml file is converted to FOSS-Report.xlsx
 - oss-pkg-info_[datetime].yaml : FOSSLight-Report.xlsx is converted to oss-pkg-info.yaml.