Update README.md

Changed "the first sheet" to "active sheet" to avoid confusion if someone saves a spreadsheet and the active worksheet is, fe. second in line.
Also, what is considered an active worksheet when there is only one worksheet in the spreadsheet.
Add an example with and without "select_sheet" option.

Author: Pytlicek

README.md

@@ -19,7 +19,7 @@ This library has 2 main features: reading a spreadsheet files and converting the
 ### - XLSX
 Use `xlsx_to_dict()` method  when converting form spreadsheets.  
 Supported file formats for spreadsheets are: .xlsx,.xlsm,.xltx,.xltm  
-Spreadsheets with multiple worksheets are supported. If no sheet is specified, the first sheet is selected.
+Spreadsheets with multiple worksheets are supported. If no sheet is specified, the active sheet is selected. If there is only one sheet, it is considered active.
 
 ```python3
 # Import the library
@@ -28,6 +28,9 @@ from sheet2dict import Worksheet
 # Create an object
 ws = Worksheet()
 
+# Convert active sheet (without specifying sheet name)
+ws.xlsx_to_dict(path='inventory.xlsx')
+
 # Convert the 'Main Warehouse' sheet of the 'inventory.xslx' spreadsheet file.
 ws.xlsx_to_dict(path='inventory.xlsx', select_sheet='Main Warehouse')