Update reproducing.md

Author: inferno-chromium

docs/reproducing.md

@@ -31,6 +31,7 @@ to replicate the exact build steps used by OSS-Fuzz and then feed the reproducer
 - *Reproduce using latest OSS-Fuzz build:* 
 
 ```bash
+$ python infra/helper.py build_fuzzers $PROJECT_NAME
 $ python infra/helper.py reproduce $PROJECT_NAME <fuzz_target_name> <testcase_path>
 ```
 
@@ -40,6 +41,7 @@ $ python infra/helper.py reproduce $PROJECT_NAME <fuzz_target_name> <testcase_pa
   E.g. for [libxml2](../projects/libxml2) project with fuzzer named `libxml2_xml_read_memory_fuzzer`, it will be: 
 
 ```bash
+$ python infra/helper.py build_fuzzers libxml2
 $ python infra/helper.py reproduce libxml2 libxml2_xml_read_memory_fuzzer ~/Downloads/testcase
 ```