docs: fix typo in Python docpage templates.md

Author: spidersouris

doc/python/templates.md

@@ -93,7 +93,7 @@ for template in ["plotly", "plotly_white", "plotly_dark", "ggplot2", "seaborn",
     fig.show()
 ```
 
-#### Specifying a default themes
+#### Specifying a default theme
 
 If a theme is not provided to a Plotly Express function or to a graph object figure, then the default theme is used. The default theme starts out as `"plotly"`, but it can be changed by setting the `plotly.io.templates.default` property to the name of a registered theme.
 
@@ -475,4 +475,4 @@ It may be useful to examine the contents and structure of the built-in templates
 import plotly.io as pio
 plotly_template = pio.templates["plotly"]
 plotly_template.layout
-```
+```