agent: Fix documentation for updating fixtures

Signed-off-by: Daiki Ueno <dueno@redhat.com>

Author: ueno

agent/src/agent.rs

@@ -300,8 +300,9 @@ mod tests {
     // files can be generated by exercising the GnuTLS session after
     // starting up the agent with:
     //
-    //   sudo target/debug/crau-agent -c agent/fixtures/agent.conf \
-    //        --library /usr/lib64/libgnutls.so.30 --user $USER:$GID \
+    //   sudo target/debug/crau-agent -c fixtures/conf/agent.conf \
+    //        --library /usr/lib64/libgnutls.so.30 \
+    //        --user $(id -n -u):$(id -n -g) \
     //        --trace-file fixtures/normal/input.cborseq \
     //        --log-file fixtures/normal/output.cborseq
     //