Fix grammar of --test-runtool-arg help text

Author: ehuss

src/librustdoc/lib.rs

@@ -520,7 +520,7 @@ fn opts() -> Vec<RustcOptGroup> {
             "",
             "test-runtool-arg",
             "",
-            "One (of possibly many) arguments to pass to the runtool",
+            "One argument (of possibly many) to pass to the runtool",
         ),
         opt(
             Unstable,

tests/run-make/rustdoc-default-output/output-default.stdout

@@ -140,7 +140,7 @@ Options:
                         documentation
         --test-runtool The tool to run tests with when building for a different target than host
 
-        --test-runtool-arg One (of possibly many) arguments to pass to the runtool
+        --test-runtool-arg One argument (of possibly many) to pass to the runtool
 
         --test-builder PATH
                         The rustc-like binary to use as the test builder