Fix typo in forms.md (#1270)

Author: JannisBush

content/develop/concepts/architecture/forms.md

@@ -67,7 +67,7 @@ st.map(df, size='size', color='color')
 
 ## User interaction
 
-If a widget is not in a form, that widget will trigger a script rerun whenever a user changes its value. For widgets with keyed input (`st.number_input`, `st.text_input`, `st.text_area`), a new value triggers a rerun when the user clicks or tabs out of the widget. A user can also submit a change by pressing `Enter` while thier cursor is active in the widget.
+If a widget is not in a form, that widget will trigger a script rerun whenever a user changes its value. For widgets with keyed input (`st.number_input`, `st.text_input`, `st.text_area`), a new value triggers a rerun when the user clicks or tabs out of the widget. A user can also submit a change by pressing `Enter` while their cursor is active in the widget.
 
 On the other hand if a widget is inside of a form, the script will not rerun when a user clicks or tabs out of that widget. For widgets inside a form, the script will rerun when the form is submitted and all widgets within the form will send their updated values to the Python backend.