fixing README bold formatting

fixing https://github.com/awslabs/git-secrets/pull/172/files

Author: PatMyron

README.rst

@@ -70,7 +70,7 @@ Homebrew (for macOS users)
 .. warning::
 
     **You're not done yet! You MUST install the git hooks for every repo that
-    you wish to use with ``git secrets --install``.**
+    you wish to use with** ``git secrets --install``.
 
 Here's a quick example of how to ensure a git repository is scanned for secrets
 on each commit::