Fix bpftrace note about environment variable (#5602)

Author: egmc

features/dtrace.xml

@@ -592,7 +592,7 @@ probe process("sapi/cli/php").provider("php").mark("request__startup") {
     path or attach by PID with <literal>-p</literal> as needed.
    </simpara>
    <simpara>
-    Make sure the target binary is built with DTrace and that the environment is configured properly.
+    Make sure the target binary is built with DTrace and the environment variable is configured properly.
     See <link linkend="features.dtrace.install">Configuring PHP for DTrace Static Probes</link> for details.
    </simpara>
    <para>