[llvm-readobj][test][NFC] - Use "--check-prefix" instead of "--check-…

…prefixes".

We have the `--check-prefixes` invocation for a single suffix.
It is cleaner to use `--check-prefix` in this case.

llvm/test/tools/llvm-readobj/ELF/needed-libs.test

@@ -58,7 +58,7 @@ ProgramHeaders:
 ## Check what we print when the dynamic string table is empty.
 # RUN: yaml2obj %s --docnum=2 -o %t2
 # RUN: llvm-readobj --needed-libs %t2 2>&1 | \
-# RUN:   FileCheck %s -DFILE=%t2 --implicit-check-not=warning: --check-prefixes=EMPTY-DYNSTR
+# RUN:   FileCheck %s -DFILE=%t2 --implicit-check-not=warning: --check-prefix=EMPTY-DYNSTR
 # RUN: llvm-readelf --needed-libs %t2 2>&1 | \
 # RUN:   FileCheck %s -DFILE=%t2 --implicit-check-not=warning: --check-prefix=EMPTY-DYNSTR