fix: Fix error messages in pull.go (#10715)

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### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
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### Documentation

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### Release notes

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Co-authored-by: Onur Cakmak <occ@occ.me>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10715
Reviewed-by: Cyborus <cyborus@disroot.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Onur Cakmak <occ@noreply.codeberg.org>
Co-committed-by: Onur Cakmak <occ@noreply.codeberg.org>

routers/api/v1/repo/pull.go

@@ -955,7 +955,7 @@ func MergePullRequest(ctx *context.APIContext) {
 	if manuallyMerged {
 		if err := pull_service.MergedManually(ctx, pr, ctx.Doer, ctx.Repo.GitRepo, form.MergeCommitID); err != nil {
 			if models.IsErrInvalidMergeStyle(err) {
-				ctx.Error(http.StatusMethodNotAllowed, "Invalid merge style", fmt.Errorf("%s is not allowed an allowed merge style for this repository", repo_model.MergeStyle(form.Do)))
+				ctx.Error(http.StatusMethodNotAllowed, "Invalid merge style", fmt.Errorf("%s is not an allowed merge style for this repository", repo_model.MergeStyle(form.Do)))
 				return
 			}
 			if strings.Contains(err.Error(), "Wrong commit ID") {
@@ -1005,7 +1005,7 @@ func MergePullRequest(ctx *context.APIContext) {
 
 	if err := pull_service.Merge(ctx, pr, ctx.Doer, ctx.Repo.GitRepo, repo_model.MergeStyle(form.Do), form.HeadCommitID, message, false); err != nil {
 		if models.IsErrInvalidMergeStyle(err) {
-			ctx.Error(http.StatusMethodNotAllowed, "Invalid merge style", fmt.Errorf("%s is not allowed an allowed merge style for this repository", repo_model.MergeStyle(form.Do)))
+			ctx.Error(http.StatusMethodNotAllowed, "Invalid merge style", fmt.Errorf("%s is not an allowed merge style for this repository", repo_model.MergeStyle(form.Do)))
 		} else if models.IsErrMergeConflicts(err) {
 			conflictError := err.(models.ErrMergeConflicts)
 			ctx.JSON(http.StatusConflict, conflictError)