config/config.example.yml: fix example syntax

As of DSpace Angular 7.2 the syntax has changed from TypeScript to
YAML.

(cherry picked from commit 9e46b53)

config/config.example.yml

@@ -292,33 +292,33 @@ themes:
   #
   # # A theme with a handle property will match the community, collection or item with the given
   # # handle, and all collections and/or items within it
-  # - name: 'custom',
-  #   handle: '10673/1233'
+  # - name: custom
+  #   handle: 10673/1233
   #
   # # A theme with a regex property will match the route using a regular expression. If it
   # # matches the route for a community or collection it will also apply to all collections
   # # and/or items within it
-  # - name: 'custom',
-  #   regex: 'collections\/e8043bc2.*'
+  # - name: custom
+  #   regex: collections\/e8043bc2.*
   #
   # # A theme with a uuid property will match the community, collection or item with the given
   # # ID, and all collections and/or items within it
-  # - name: 'custom',
-  #   uuid: '0958c910-2037-42a9-81c7-dca80e3892b4'
+  # - name: custom
+  #   uuid: 0958c910-2037-42a9-81c7-dca80e3892b4
   #
   # # The extends property specifies an ancestor theme (by name). Whenever a themed component is not found
   # # in the current theme, its ancestor theme(s) will be checked recursively before falling back to default.
-  # - name: 'custom-A',
-  #   extends: 'custom-B',
+  # - name: custom-A
+  #   extends: custom-B
   #   # Any of the matching properties above can be used
-  #   handle: '10673/34'
+  #   handle: 10673/34
   #
-  # - name: 'custom-B',
-  #   extends: 'custom',
-  #   handle: '10673/12'
+  # - name: custom-B
+  #   extends: custom
+  #   handle: 10673/12
   #
   # # A theme with only a name will match every route
-  # name: 'custom'
+  # name: custom
   #
   # # This theme will use the default bootstrap styling for DSpace components
   # - name: BASE_THEME_NAME