Update flask.asciidoc
Shell command fails with certain shells (for ex. zsh), adding quotes.
@@ -12,7 +12,7 @@ Install the Elastic APM agent using pip:
[source,bash]
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-$ pip install elastic-apm[flask]
+$ pip install "elastic-apm[flask]"
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or add `elastic-apm[flask]` to your project's `requirements.txt` file.
