Fix "assertion batch" anchor (dafny-lang#5096)

### Description
This is a fix for the broken "assertion batch" anchor -- an example use
of it is below:


![assertion-batch-link](https://github.com/dafny-lang/dafny/assets/12002672/e34d1c7e-e5d9-4314-806e-41e3294f0a1e)

The anchor was originally
`#sec-verification-attributes-on-assert-statements` which currently
doesn't exist. The closest existing anchor appears to be
[`#sec-verification-attributes-on-assertions`](https://dafny.org/dafny/DafnyRef/DafnyRef#sec-verification-attributes-on-assertions)
but I think linking to the section for the anchor
[`#sec-assertion-batches`](https://dafny.org/dafny/DafnyRef/DafnyRef#sec-assertion-batches)
is better since that section introduces the _assertion batch_ concept.

### How has this been tested?
I have not compiled the Dafny VSCode extension locally, but this PR
changes tests and the CI should be green.

----

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/DafnyLanguageServer.Test/Lookup/HoverVerificationTest.cs

@@ -137,7 +137,7 @@ await AssertVerificationHoverMatches(documentItem, (0, 36),
         @"**Verification performance metrics for method `f`**:
 
 - Total resource usage: ??? RU  
-- Most costly [assertion batches](https://dafny-lang.github.io/dafny/DafnyRef/DafnyRef#sec-verification-attributes-on-assert-statements):  
+- Most costly [assertion batches](https://dafny-lang.github.io/dafny/DafnyRef/DafnyRef#sec-assertion-batches):  
   - #???/??? with 1 assertion  at line ???, ??? RU  
   - #???/??? with 1 assertion  at line ???, ??? RU  "
       );
@@ -223,7 +223,7 @@ await AssertVerificationHoverMatches(documentItem, (0, 36),
         @"**Verification performance metrics for function `f`**:
 
 - Total resource usage: ??? RU  
-- Most costly [assertion batches](https://dafny-lang.github.io/dafny/DafnyRef/DafnyRef#sec-verification-attributes-on-assert-statements):  
+- Most costly [assertion batches](https://dafny-lang.github.io/dafny/DafnyRef/DafnyRef#sec-assertion-batches):  
   - #???/2 with 2 assertions at line ???, ??? RU  
   - #???/2 with 2 assertions at line ???, ??? RU"
       );

Source/DafnyLanguageServer/Handlers/DafnyHoverHandler.cs

@@ -441,7 +441,7 @@ public static string FormatResourceCount(long nodeResourceCount) {
     }
 
     private static string AddAssertionBatchDocumentation(string batchReference) {
-      return $"[{batchReference}](https://dafny-lang.github.io/dafny/DafnyRef/DafnyRef#sec-verification-attributes-on-assert-statements)";
+      return $"[{batchReference}](https://dafny-lang.github.io/dafny/DafnyRef/DafnyRef#sec-assertion-batches)";
     }
 
     private static Hover CreateMarkdownHover(string information) {