[bugfix/frontend] web/template/page_header.tmpl: use singular verb if necessary (#4600)

# Description

If there's just one user, say "who has written x posts", otherwise say "who have written x posts".

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first two checkboxes must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have not used so-called 'AI' to create the proposed changes.
- [ ] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [ ] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4600
Co-authored-by: Roberto Ricci <io@r-ricci.it>
Co-committed-by: Roberto Ricci <io@r-ricci.it>

web/template/page_header.tmpl

@@ -52,9 +52,9 @@ Instance Logo
 {{- define "strapUsers" -}}
 {{- with deref .instance.Stats.user_count -}}
     {{- if eq . 1 -}}
-        <span class="count">{{- . -}}</span> user
+        <span class="count">{{- . -}}</span> user who has written
     {{- else -}}
-        <span class="count">{{- . -}}</span> users
+        <span class="count">{{- . -}}</span> users who have written
     {{- end -}}
 {{- end -}}
 {{- end -}}
@@ -101,6 +101,6 @@ Instance Logo
     <h1>{{- .instance.Title -}}</h1>
 </a>
 {{- if .showStrap }}
-<aside>home to {{ template "strapUsers" . }} who have written {{ template "strapPosts" . }}; knows of {{ template "strapInstances" . }}</aside>
+<aside>home to {{ template "strapUsers" . }} {{ template "strapPosts" . }}; knows of {{ template "strapInstances" . }}</aside>
+{{- end }}
 {{- end }}
-{{- end }}