Minor doc fix (pytorch#3339)

Summary:
X-link: facebookresearch/FBGEMM#433

- Minor doc fix

Pull Request resolved: pytorch#3339

Reviewed By: sryap

Differential Revision: D65584708

Pulled By: q10

fbshipit-source-id: c0d30277bc91809cad0272a64e398d97ac096ae2

fbgemm_gpu/docs/src/fbgemm_gpu-development/BuildInstructions.rst

@@ -470,7 +470,7 @@ Clone the repo along with its submodules, and install the
 
   # Install additional required packages for building and testing
   cd fbgemm_${FBGEMM_VERSION}/fbgemm_gpu
-  pip install requirements.txt
+  pip install -r requirements.txt
 
 The Build Process
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