docs: fix code snippets that do not compile

[meliharik]

Four documentation snippets do not parse as written. Each was verified with dart format --output=none: the block fails to parse before the change and parses after it.

Dartdoc — unterminated string

packages/firebase_core/firebase_core_web/lib/src/firebase_app_web.dart:14
packages/firebase_core/firebase_core_platform_interface/lib/src/method_channel/method_channel_firebase_app.dart:14

-/// Firebase.app('SecondaryApp`);
+/// Firebase.app('SecondaryApp');

Both files close the string with a backtick instead of '. Same line, same typo, presumably copied between the two.

The web file also says "call the the app method" one line above, fixed here too.

docs/perf-mon/_custom-code-traces.md:154 — doubled paren

-customTrace.putAttribute(("email", user.getEmailAddress());  // Don't do this!
+customTrace.putAttribute("email", user.getEmailAddress());  // Don't do this!

To be explicit about intent: the "Don't do this!" warning is about putting personally identifiable information in an attribute, not about the syntax — the "OK" example seven lines above is putAttribute("experiment", "A"); with a single paren. The extra ( is unintended.

docs/auth/email-link-auth.md:102 — leftover closing

     .then((value) => print('Successfully sent email verification'));
-});

The statement already terminates with ; on the previous line; the }); has no matching opener inside the block.

How I found them

I extracted every fenced code block and /// doc comment across the repo and checked paren/bracket balance outside strings and comments, then read each hit in context to separate real typos from intentional fragments. Docs-only change — no behaviour, no tests affected.

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[meliharik]

@googlebot I signed it!

[SelaseKay]

Hi @meliharik, thanks for the contribution. LGTM!