Fix incorrect documentation for syntax directory (#1901)

Currently the Configuration docs says to load syntax files into
config.toml [markdown] extra_syntaxes. However, ever since commit
23064f5 (released in Zola v0.15.0), the extra_syntaxes property was
replaced by extra_syntaxes_and_themes, and used as both syntax and color
theme search paths. Following the docs and trying to set the
extra_syntaxes property does nothing, and #1723 ran into this issue.

Change the docs to consistently reference extra_syntaxes_and_themes.

docs/content/documentation/getting-started/configuration.md

@@ -93,8 +93,8 @@ build_search_index = false
 # When set to "true", all code blocks are highlighted.
 highlight_code = false
 
-# A list of directories used to search for additional `.sublime-syntax` files.
-extra_syntaxes = []
+# A list of directories used to search for additional `.sublime-syntax` and `.tmTheme` files.
+extra_syntaxes_and_themes = []
 
 # The theme to use for code highlighting.
 # See below for list of allowed values.