Fix MkDocs encountered an error parsing the configuration file (#143)

Author: Nirus2000

config/en/mkdocs.yml

@@ -121,7 +121,7 @@ nav:
       - Portfolio Performance terminology: concepts/portfolio-performance-terminology.md
       - Financial terminology: concepts/financial-terminology.md
       - System overview: concepts/system-overview.md
-	  	  - Example: concepts/system-overview-example.md
+          - Example: concepts/system-overview-example.md
       - Reporting period: concepts/reporting-period.md
       - Purchase value: concepts/purchase-value.md
       - Performance: