Properly quote description for ad metric

This is pedantic, but strictly something called <provider-name> is considered an HTML tag
unless it's in a code block (backticks). 

See mozilla/glean-dictionary#549 and mozilla/glean-dictionary#497. I'm going to fix this upstream
but figured I might as well file a PR here to fix the underlying issue.

app/metrics.yaml

@@ -3949,7 +3949,7 @@ browser.search:
     type: labeled_counter
     description: |
       Records counts of SERP pages with adverts displayed.
-      The key format is ‘<provider-name>’.
+      The key format is `<provider-name>`.
     send_in_pings:
       - metrics
     bugs:
@@ -3966,7 +3966,7 @@ browser.search:
     type: labeled_counter
     description: |
       Records clicks of adverts on SERP pages.
-      The key format is ‘<provider-name>’.
+      The key format is `<provider-name>`.
     send_in_pings:
       - metrics
     bugs: